Under the old political rules, Mitt Romney arrived in South Carolina this week the prohibitive Republican front-runner: flush with cash, awash in endorsements from a party establishment starting to coalesce behind him, and buoyed by victories in Iowa and New Hampshire.

But as Romney is quickly learning, those rules no longer apply.

Romney's carefully tended network of Republican donors has been rendered functionally irrelevant by "super PACs," through which a handful of wealthy individuals are financing a multimillion-dollar advertising barrage to assail his record and prop up his opponents.

And Romney's victories in Iowa and New Hampshire have netted him just a handful of the delegates he needs to become the nominee, thanks to the party's decision last year to lengthen the nominating process by shifting more winner-take-all contests to the end of the primary season.

As a result, Romney's remaining opponents have little incentive to drop out, knowing that their support from super PACs and Internet contributions from grass-roots supporters can keep them in the race long after they would have remained viable in earlier eras, potentially draining money and delegates away from Romney even as he lurches toward the nomination.

Since the contests began, only one candidate, Rep. Michele Bachmann, has pulled out, while two others, Newt Gingrich and Gov. Rick Perry, have failed to break into the top three in Iowa or New Hampshire.

"I think everybody in the race right now has a legitimate shot," Romney said at a news conference in South Carolina on Thursday. "And I wish them all well."

The debut of candidate-aligned super PACs in the Republican primaries has further upended a party already struggling to reconcile its restive tea party-inspired grass-roots with a traditional party apparatus that is rapidly losing whatever control it once exerted over the process.

On Thursday, even as dozens of wealthy Republicans gathered at a Palm Beach, Fla., fundraiser and raised an estimated $1.5 million for Romney the old-fashioned way, a super PAC backing Gingrich began buying half-hour blocks of advertising time in South Carolina, with preparations to broadcast a scathing video about Romney's record as the founder of Bain Capital.

The group, Winning Our Future, is armed with $5 million from just a single Gingrich ally, the casino magnate Sheldon Adelson. And Gingrich signaled on Thursday that he would continue his own assaults on Romney's private equity days, even as other prominent Republicans – including Rudy Giuliani and Sen. Jim DeMint of South Carolina, a tea party favorite – called on him to cease.

Asked if Winning Our Future was keeping him in the race, Gingrich said that "it just offsets the other PACs."

He is not necessarily wrong.

The Red, White and Blue Fund, an outside group that is promoting the candidacy of Rick Santorum, began advertising in South Carolina before the New Hampshire primary, softening the negative effects of Santorum's potentially campaign killing – at least by the old rules – fourth-place finish there on Tuesday.

On Thursday, the group, which was seeded with a major contribution from Foster Friess, a wealthy mutual fund executive, announced a new investment of $600,000 in television advertising in South Carolina. The group had its largest fundraising day on Wednesday, officials said, allowing it to expand its reach into all television markets in the state.

In a brief interview on Thursday, Santorum said that he welcomed the support and acknowledged that his allies had kept his candidacy afloat.

"We certainly don't have the kind of money that these other super PACs have," Santorum said after a stop in Hilton Head, S.C., "but the modest amount that they're able to put out there will hopefully be a help to us."

While Jon Huntsman has had too little campaign cash to afford substantial advertising, a super PAC supporting him, Our Destiny, helped keep him afloat through New Hampshire and is expected to weigh in for him to some degree in South Carolina.

The group has received hundreds of thousands of dollars from Huntsman's father, a billionaire industrialist.

Officials with Huntsman's campaign have conceded that without the help of the super PAC supporting him, they would have had a much harder time competing. But John Weaver, the chief political strategist for Huntsman, said having Our Destiny's help was "not a clean shot" because it left the campaign's strategy in the hands of people it is legally prohibited from coordinating directly with.

The outside help for his rivals may, of course, prove to have an upside for Romney: By keeping a half-dozen different candidates viable, the super PACs may inadvertently be making it impossible for one of them to emerge as the alternative to him.

But the change in the delegate-awarding process means that if Romney continues to win states only with pluralities, he could face a long nominating fight, as other candidates pick up delegates under new rules that increase the number of states that award delegates based on vote share.

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