"I always say politicians are cowards, and they really are," Stephen Moore told me recently. "We say we're going to run someone against them, and they start wetting their pants."

Moore, the president of a group of zealous economic conservatives known as the Club for Growth, was talking about Arlen Specter, a giant of the United States Senate and the only Republican moderate in the Senate leadership. Specter is running for a fifth term next year in Pennsylvania, but he now finds himself facing an unexpected, potentially serious primary challenge from the party's right flank. That challenge, from a brash conservative congressman from industrial Allentown named Patrick Toomey, is being engineered by the Club for Growth, whose 10,000 members, most of them gray-suited bankers and businessmen, seem to be on a mission to banish taxes from the earth. Moore has vowed revenge on Republican incumbents who don't worship at his antitax altar -- he calls them "Republicans in Name Only," or "Rinos" -- and unseating Specter, he says, is his top priority.

Although Specter is a powerful committee chairman and can count on the strong support of the White House, he is clearly anxious; he is already spending much of his time shaking hands back in Pennsylvania, and he has called some members of the club himself to plead his case. A few months ago, Specter even invited Moore over to his Capitol office for a chat. A masterly politician, Specter gave it all the charm he could muster, graciously showing Moore his trove of family photos before launching into a defense of his voting record, which, he rightly pointed out, is broadly more conservative than Toomey's, according to National Journal's ratings. Specter is, after all, the man who got Clarence Thomas confirmed, and he has long supported the balanced-budget amendment and, for that matter, the flat tax.

To Specter's astonishment, however, Moore, a nerdy 43-year-old economist with an affable, self-mocking laugh, didn't seem to care much about Specter's record. It was simple, Moore said: even though Specter eventually voted for President Bush's $1.3 trillion tax cut, Moore could not forgive him for first voting to trim the president's original, bigger tax-cut proposal by $250 billion so that the money could be spent on education.

For his part, Specter came away from his meeting with Moore feeling somewhat bewildered by what was happening inside his own party. He took note when Moore later told a reporter that he wanted to beat Specter because having "a major scalp on the wall" would make the Club for Growth more intimidating to other Republicans. (Moore said the same thing to me when I interviewed him in his office, motioning to a spot near his desk, as if Specter's scalp might actually hang there someday.) "I have been in public life since I became an assistant D.A. in 1959," Specter told me. "And I've never heard talk like that."

Outside of Washington, only Stephen Moore's relatives have ever heard of him -- and even some of them would probably have to think on it a while. But in this era after campaign-finance reform, as the fund-raising dominance of the two parties diminishes, it's the people you've never heard of who will change the course of politics, and Moore seems bent on taking a hammer to George W. Bush's carefully sculptured majority party. Last spring, Moore attacked two Republican senators who were resisting the latest tax cut: George Voinovich of Ohio and Olympia Snowe of Maine. He ran ads in each of their states in which he compared them with the French president, Jacques Chirac. That was too much for Karl Rove, the president's political enforcer, who called the ads "stupid" and "counterproductive."

Moore was unimpressed. "Whether I'm pals with Karl Rove is not really of interest to me, to be honest with you," he told me.

This kind of open rebellion is unheard of in Bush's Washington, where party loyalty gets confused with moral rectitude, and where Republicans generally speak with a kind of bland, Orwellian unanimity. The criticism is especially surprising since it comes from activists on the party's economic fringe, at what looks for all the world like their moment of triumph, having just won the largest tax break since Reagan. You could understand if religious conservatives were the ones agitating, given their discontent over the recent Supreme Court decisions on affirmative action and gay rights. But what could the radical supply-siders possibly have to complain about?

Back in the early 1980's, a group of about 30 wealthy money managers in Manhattan, all of them Reagan-loving, tax-hating conservatives, began holding a monthly political meeting that they fashioned on a venture-capital model. The "investors" would invite a political candidate running for state or national office in to explain why he or she would be a strong fiscal conservative, and if they liked what they heard, they would write checks on the spot. Bravely casting aside the obvious hair-transplant jokes, they called themselves the Political Club for Growth.

As years passed, however, the club's members began to see that candidates would say anything to get a pile of checks, only to ignore their promises to the club once they got elected. The supply-siders needed a new approach.

One of the club's regular attendees was Stephen Moore. He wasn't rich, but as a scholar at the libertarian Cato Institute and a former Republican aide on Capitol Hill, he was considered an expert on the federal budget. Moore was intrigued by the success of Emily's List, a pro-choice group that had developed serious clout among Democrats by helping female candidates in primaries, where a relatively small amount of money could have a huge impact. Emily's List amassed power by '"bundling" individual donations and passing them on to selected campaigns. In 1998, Moore persuaded his wealthy backers -- they included the philanthropist and financier Richard Gilder and Thomas L. Rhodes, president of The National Review -- to let him remake the Club for Growth in the same mold.

The club's agenda hasn't changed much since those formative days of the Reagan administration. Like all good supply-siders, Moore and his members hold that the best way to stimulate economic expansion is for government to drastically reduce the amount of money it collects from its citizens. This practice, as recent history shows, has a tendency to create budget deficits, but the supply-siders say that's all right, because deficits force government to scale back spending on inefficient programs.

"Growth is the answer to all our problems," Moore told me over lunch at Morton's, a favorite Republican hangout. "Poverty, income disparity, disease -- all of these things can be alleviated through rapid income growth."

For a guy who talks like a professor, Moore has proved himself to be a talented and ruthless self-promoter, establishing the Club for Growth as an irrepressible presence in Washington. Moore has intervened in open primaries to help elect several current congressmen -- Patrick Toomey among them -- who are now fiercely loyal to his agenda. His close relationship with these congressmen and a handful of senators has, in turn, enhanced his power on the Hill. He's a regular visitor in the office of Tom DeLay, the Republican house majority leader, whose political-action committee has also contributed heavily to the club.

Republicans who disdain the Club for Growth are quick to point out that for all Moore's chesty threats, the club has yet to actually beat an incumbent, and its overall won-lost record is decidedly mixed. "At the end of the day, these guys have more dollars than political sense" is the way Scott Reed, a Republican strategist, put it.

In politics, however, there are times when the facts aren't as relevant as the perception. The club may not, as yet, have unseated any Republicans, but the looming certainty that it could -- and will go to any length to succeed -- terrifies vulnerable incumbents. A few years ago, it would have been unthinkable for moderates in both parties to talk of a $350 billion tax cut at a time of deficit spending as a "compromise," and yet that is precisely what just happened in Washington. Moore's tactics have helped tilt the terms of debate in the Capitol.

During the 2002 campaign cycle, in addition to spending several million dollars on polling and ads, the Club for Growth collected at least $3.2 million in "bundled" checks for its chosen candidates, according to the Center for Responsive Politics. One of the lucky recipients was David Fischer, a Maryland lawyer the club backed in an unsuccessful bid to unseat Congressman Wayne Gilchrest. Fischer described to me how the club sent him about $225,000, roughly three-quarters of the total cash donated to his campaign.

"I felt like one of those congressman back in Abscam, when they used to get those packages of money," Fischer told me. "We were taking in 25 or 30 grand a day."

To be the target of the Club for Growth is a different experience altogether. Just ask Sherwood Boehlert, an 11-term moderate congressman from New York who was singled out by the club last year and barely survived. "What we didn't see coming was the last 10-day onslaught of money, manpower and mailings," Boehlert says. "And those mailings -- if I believed that about a candidate, I wouldn't have voted for me either."

Like some other Republicans at whom the club has taken aim, Boehlert isn't really sure why. He voted for both Bush tax cuts, along with rollbacks of the marriage tax and the estate tax. When I mentioned this to Moore, he was unmoved. "He's been one of the most liberal Republicans in the House," Moore said, adding that conservatives consider Boehlert to be a big spender.

"We're not only about tax cuts," Moore reminded me, and it's true: the club claims to judge candidates not just by their commitment to tax cuts, but also by their determination to rein in government waste. (The club is "agnostic" on social issues, to such an extent that Moore has banned "the A word" -- abortion -- from its meetings.)

But for an organization that claims to be so ideologically pure, the club doesn't always apply these principles consistently. As Moore readily admits, spending has multiplied like a virus in Washington under Bush and the Republican Congress, but while the club gleefully goes after Boehlert and other moderates on taxes, it has yet to take aim at a single conservative for going soft on spending. Sometimes, in fact, the club isn't at all concerned with a candidate's fiscal ideology -- like, for instance, when that candidate's name is Arnold Schwarzenegger. Earlier this summer, Moore held an event with the Terminator as he considered entering the California governor's race on the recall ballot. A few weeks later, at a staff meeting, Moore and his deputies discussed the actor's potential candidacy.

"Where is he on policy?" one staff member wanted to know.

"Oh, he's terrible," Moore said. "Horrible. He says he's a fiscal conservative, but. . . . " He trailed off in laughter. If Schwarzenegger would just agree to back a flat-tax proposal and to rule out any new taxes, Moore said, the club would get behind him anyway.

This kind of inconsistency -- "hypocrisy" would be the less charitable word -- has led critics of the club to conclude, not unreasonably, that its talk about cutting taxes and slashing spending obscures a less lofty agenda. Moore wants that scalp on his wall, and the easiest way to get it is to single out moderates like Specter and Boehlert, because moderates are always vulnerable in Republican primaries.

"They go after the low-hanging fruit," says Sarah Chamberlain Resnick, who runs the Republican Main Street Partnership. Resnick's group, a coalition of Republican moderates, is vowing to raise at least $5 million to fight Moore in critical states next year -- which suggests, incredibly, that a president poised for coronation could instead find his party drawn into civil unrest.

The Bush White House has created such an aura of discipline that Republicans in Washington often behave as if their offices are bugged, faithfully mouthing the message of the day. And so the most intriguing question about the Club for Growth, one that divides Republicans in Washington, is whether Moore is the pest that the administration just can't seem to exterminate, or actually a secret ally of the White House. Put another way, if Karl Rove is really so angry at Moore's antics, why does the club still exist?

For much of the first two years of the administration, Moore was part of the right-wing cabal that administration officials would consult on a regular basis. But Moore proved himself disloyal by publicly criticizing Bush and opposing some of his appointments. "I don't really like the Bush people very much," Moore says. "I was never part of a fraternity or anything like that, and the Bush White House is like a club."

Rove has good reason to wish the Club for Growth would go away. It will be hard to achieve his goal of expanding the G.O.P.'s margin in the House and Senate if Republicans face divisive primary fights in critical swing states. It's not surprising, then, that Rove and his deputies have called some of Moore's backers to complain about the club's tactics. And Rove has made it clear that the White House intends to provide Specter with the muscle he needs to fend off Toomey's challenge.

At the same time, however, the club serves a useful purpose for the Bush administration by driving vulnerable moderates into the warm embrace of the White House. This makes it much easier for the administration to make deals on the Hill. Think about it: if you're a moderate congressman, you might be inclined to vote against the latest Bush tax cut. But then here's Steve Moore, announcing that he won't rest until you're retired and teaching civics at a community college. You have a choice: you can either vote against the tax cut and take on a club-backed opponent all by yourself, or you can cut a deal with Rove, who will send in the president to campaign for you, as long as you give him your vote on the tax cut. In this Machiavellian way, the club is the perfect foil for the White House, creating leverage where it might otherwise not exist.

The one thing Rove won't tolerate, apparently, is Moore's taking credit for the president's legislative victories. I sent Rove an e-mail message with several questions regarding the Club for Growth. He ignored every question except one -- when I asked if the Club for Growth had, in fact, helped secure Senator Voinovich's vote on the tax cut with its vicious ad campaign, as Moore said it did. "The ads were completely ineffective," Rove wrote back, "and what won Voinovich's support was the constant lobbying and cajoling of his Senate colleagues, his Ohio constituents and the president of the United States."

On any given day in the Club for Growth's K Street office, another smartly dressed, little-known Republican congressional candidate may come traipsing through to beseech Moore for his support. These meetings often have the feel of job interviews, with the eager candidate sunk into a beaten office chair while Moore, seated at his paper-strewn desk, flips lazily through the applicant's resume.

The first time I visited the offices of the Club for Growth, in June, Moore was just wrapping up a conversation with Virginia Foxx, a matronly state senator from North Carolina who is now running for a seat in Congress.

"Is it Foxx with two x's?" Moore asked absently. The candidate nodded and pointed to her two earrings -- an x in each ear.

"Now, someone called me this past week and said all this stuff about you voting for all these tax increases," Moore said.

"No, no," Foxx replied quickly, shaking her head. "It was just that one I told you about already. The occupancy tax."

"You sure?"

"Yes, yes," Foxx assured him. She left with a check for $1,000 but no guarantee of support.

That candidates are willing to endure Moore's obvious contempt for them -- "Aren't politicians loathsome?" he asked me once with a pained expression, as if we were discussing head lice -- indicates that Moore is becoming something of a kingmaker inside the party. And this, ultimately, is what the Club for Growth is after: to be the interest group that controls Republican politics.

"The only group that has been jealous of us -- I don't know if 'jealous' is the right word, or maybe 'resentful' -- is the party itself," Moore said. "They think we're replacing the party, and that is our goal. We want to take over the party's fund-raising. We want it to be, in 10 years, that no one can win a Senate or a House seat without the support of the Club for Growth."

It may sound Napoleonic, this business about taking over the nation's dominant party, but it might not be as delusional as it sounds. Moore understands, shrewdly, that the old order of political power is fast eroding. Party loyalty has been declining for years, and voter turnout remains strikingly low, meaning that elections increasingly revolve around the kind of single-issue voters that give interest groups their power. And under the new McCain-Feingold campaign finance law, the two parties can no longer spend unlimited amounts of soft money, which takes away their chief fund-raising advantage over outside groups like Moore's.

While it's pretty unlikely that the Club for Growth will be running Republican politics anytime soon, it's not a stretch to suggest that in 10 years it will be one of several groups, each with its own extreme agenda, that will dominate the business of campaigns.

To that end, Moore keeps looking for the one big score. On the day he met with Foxx, Moore giddily waved a rubber-banded stack of envelopes at me. It was the week's accumulation of checks for Toomey, soon to be sent over to his campaign in a Federal Express envelope. "This will shatter all our records," Moore said. It's a long way to next year's election, and despite the club's best efforts, most political experts think Arlen Specter will emerge with his Senate seat. Sooner or later, though, Moore will most likely have somebody's scalp hanging on that wall.

Matt Bai is a contributing writer for the magazine.

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