'I'd rather lose with Pat Toomey than win with Arlen Specter any day." That's South Carolina Republican Jim DeMint defending his Senate Conservatives Fund, a new PAC that has taken Washington by storm.

The fund-raising group has already helped eight underdog Reaganite candidates win Republican Senate primaries this year. In two years, the fund has raised and spent nearly $2 million from nearly 50,000 individual contributors.

Zina Saunders

Mr. DeMint's mission is to bring more Jim DeMints to the Senate—that is, people with an unfailing antagonism to big government. But his string of victories, often against establishment candidates, has many of his Republican colleagues grumbling. They say Mr. DeMint is pushing candidates through the primaries who are too far to the right to take back vulnerable seats from Democrats in November. Former Senate Majority Leader Trent Lott recently spoke for many in the party when he said it didn't need anymore "Jim DeMint disciples."

Over the past five years, Mr. DeMint has established himself as the pre-eminent conservative in Congress—he has a near perfect National Taxpayer Union rating—with Tom Coburn of Oklahoma a close second. As we eat lunch at Mr. DeMint's favorite restaurant in his hometown of Greenville, our conversation is often interrupted by well-wishers thrilled to see their senator in person and all with pretty much the same message: "Keep fighting those big spenders."

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Mr. DeMint savors his PAC's most recent victory in Colorado, where $141,000 in radio ads and direct contributions helped Ken Buck defeat Jane Norton, the choice of Republican Senatorial Committee Chairman John Cornyn. Mr. DeMint grouses that Mr. Buck was never even presented to his colleagues as a "viable alternative, which seemed unfair." He adds, only half-kiddingly, that what did in Ms. Norton was that she was "endorsed by 25 Republican senators, which made her the establishment candidate." These days, that's the kiss of death.

Other victors helped by Mr. DeMint include Rand Paul of Kentucky, Mr. Toomey of Pennsylvania and Mike Lee of Utah (but only after incumbent Sen. Robert Bennett was knocked out at the Utah GOP convention). He says his goal is to raise $5 million this cycle. That's a pittance in big-money politics, but Mr. DeMint's strategic, targeted spending has flipped more races than even he thought possible.

"I'm not a kingmaker," he insists, even though that's precisely what many political pros call him. "And these guys don't want to be kings. We've got too many kings in Washington already."

A year ago, Mr. DeMint was demoralized and considered not running for re-election. "Why do I want to beat my head against the wall for another six years?" he recalls thinking. "I called my wife in December and said I'm ready to give it up. I'm not making any headway and most of my own colleagues are against me up here. I don't even like playing a contentious role. I like to be a strategic policy guy."

How many Republicans can be counted on to follow him into these budget battles? "Well, there's Coburn, who has got the courage to go out and make a scene on the floor or to stand up in a conference meeting and stand up to the appropriators. We don't have anyone else." Hence the PAC, which he says is the culmination of years of frustration from working within the system to fix Washington.

"When I got to Congress in 1999, instead of working on the reforms that I ran on—wealth-creating personal accounts and individually owned health insurance and some simple tax, the things that I thought all of us believed in—instead we worked on redistricting and getting the vulnerables on the right committees and getting earmarks to the people who needed them. Everything was about numbers and electing more Republicans. We'd always promise to get to the principles later." He shakes his head: "I just thought maybe there's something I don't understand."

He even admits: "I played along for a while. I asked for earmarks. I thought that following [longtime South Carolina Sens.] Fritz Hollings and Strom Thurmond, part of my job was getting a fair share for South Carolina. But we spent most of each year directing appropriations for parochial projects and it undermined our brand as Republicans and our entire anti-big government agenda."

In 2006 and 2007, he tried to fund raise for the GOP and the official Senate campaign committee. "I discovered that people were just so frustrated with the Republicans. I was over there at the Senate committee making fund-raising calls and so many people were saying, 'I'm not giving you guys another dime until you start acting like Republicans.' That's when I got the idea of starting a committee to just help conservative candidates."

His frustration boiled over in 2009 when the Republican Senatorial Committee endorsed Arlen Specter and Charlie Crist, neither of whom is a Republican one year later. Mr. DeMint was the first major political figure to endorse Marco Rubio against Mr. Crist in Florida. Although Mr. Rubio is embraced now as a rising star of the Republican Party, at the time people laughed. "Yeah, many of my Senate colleagues weren't too happy. I think in the beginning they thought what I was doing was such a small thing that it would not threaten them." How wrong they were.

As the midterms approach, Mr. DeMint is also up for re-election, but his hapless Democratic opponent, Alvin Greene, is fighting a felony pornography charge. So most of his focus is on the five to eight stalwart conservatives who might be joining him in the Senate next year, and in the fight for limited government.

He tells me the story of a meeting that Republican senators had with Ron Johnson, the businessman and GOP senatorial candidate in Wisconsin. "He was asked why he's running for Senate and he stood up, and I hadn't met him yet, he looked straight at me and he said, 'I just want to quote DeMint here. I'm coming here to join the fight, not the club.' And I laughed and said, 'Well, this is the club.'"

That club got disrupted further last week when incumbent Sen. Lisa Murkowski apparently lost to firebrand challenger Joe Miller in Alaska's Republican primary. (Absentee votes are still being counted.) "It's encouraging to me what happened in Alaska with Miller," Mr. DeMint told me yesterday. "It should be a wake-up call to Republicans that politicians who go to Washington to bring home the bacon aren't wanted—even in a state like Alaska that has gotten so much pork under senators like Ted Stevens. Voters are saying 'We're not willing to bankrupt the country to benefit ourselves.'"

The Alaska race highlights the tensions that are taking hold within the Republican Party. Can moderates and conservatives co-exist? At the moment, it seems that such unity would be necessary for taking back majorities from the Democrats. Mr. DeMint believes that "sure, numbers matter, I understand that, but not if we have to cave in our principles."

So what should the Republicans' top priority be if they take back the House, the Senate or both this year? "You need to start by putting a cap on spending."

Next he says, "we may not be able to repeal ObamaCare, but we can cut off the funding." Will they really? "Yes, if you have a wave of new people coming in—they've all campaigned on it."

Then, he says, sell Chrysler and GM. "It doesn't matter how much money we lose; let's get out of it." He also wants to privatize Fannie Mae and Freddie Mac so we can "get out of running the housing industry." He also wants to see a low-rate flat income or consumption tax.

His other personal crusade is to end earmarks. He thinks Republicans can reconnect with voters by doing away with pork-barrel spending. "Mainstream America that doesn't care about politics knows we need to stop. I had a group of 400 pastors stand up and applaud when I said we've got to stop earmarks." They understand there is something immoral and corrupting about wasting taxpayer money.

But in a $3.7 trillion budget, aren't earmarks trivial? He scoffs: "They always say, it's just a small amount of money, but earmarks always enlarge our budget and buy votes so that massive bills can go through." Members of Congress haven't been able to fight against obese budgets, he says, because "when we direct money back home through earmarks, it makes us complicit in the spending process. It's a killer."

I ask what so many voters are pondering: If Republicans win this fall, will they have learned the lessons from the overspending and corruption that got them tossed out in 2006 and 2008?

"In the House, John Boehner and the Republicans get it," Mr. DeMint says. He's not so sure about the Senate. "I think we're in danger of doing the same thing we did before, where a lot of young conservatives come in who have been out there campaigning on the right issues, but then all the senior guys take control of the committees and it's business as usual."

He warns: "This may be our last chance with voters, because if we're given the majority . . . and don't reform Washington, everybody is going to say, 'What's wrong with these guys? We need a third party.'"

He says he has more faith in voters than in the people they elect. "I'm getting optimistic. I think, as I talk to people around the country—they seem to get it. They want a return to those things that made America different and great. They understand that what the government has done is so harmful, in terms of spending and takeovers, the debt, it has made people who are not normally political and not generally interested in it alarmed.

"What makes the difference for me is feeling like I'm really giving a voice to people who care about what happens to our country." That is why Jim DeMint, for better or worse, has suddenly become a major political force.

Mr. Moore is senior economics writer for The Wall Street Journal editorial page.