Harold C. Simmons, a billionaire corporate raider from Texas, pulled out his checkbook on Jan. 13 and gave $100,000 to a super PAC backing Mitt Romney, then donated $5 million more to another PAC stacked with Romney confidants.

But 11 days later, Simmons doled out $500,000 to a super PAC devoted to Newt Gingrich, who had just trounced Romney in the Republican presidential primary in South Carolina.

(See a full list of the biggest donors to the super PACs backing each presidential candidate.)

Simmons is part of a rarefied group of millionaires and billionaires acting as kingmakers in the GOP contest, often helping to decide, with a simple transfer of money, which candidate might survive another day.

Although many of these mega-donors have long participated in politics, none were able to wield the kind of influence now possible under loosened campaign finance regulations, which allow super PACs and other outside groups to spend unlimited amounts on political races.

In January, just five donors gave a total of $19 million, a quarter of the money raised for the presidential race that month, according to a Washington Post analysis of new contribution data filed this week. Overall, 23 people have directed about $54 million to super PACs this cycle, helping to bankroll a tide of negative ads in primary-contest states.

The dominance of a handful of well-to-do donors has suddenly reshaped campaign finance, but it could also pose a political risk to candidates in both parties at a time of economic distress, particularly as President Obama and his Republican rivals debate issues relating to tax fairness and income inequality ahead of the November election.

“I’m against very wealthy people attempting to or influencing elections,” casino mogul Sheldon Adelson, who is funding a pro-

Gingrich super PAC, said in an interview published this week in Forbes magazine. “But as long as it’s doable, I’m going to do it.”

The biggest super PAC donors represent a cross section of the nation’s elite, from financiers backing Romney — such as hedge fund kings John Paulson and Julian Robertson — to ideologically driven contributors such as Adelson, who has said he supports Gingrich because of his hawkish views on Israel.

Many of the big donors don’t confine themselves to a single gift or group, returning to their checkbooks again and again.

PayPal co-founder Peter Thiel wrote four checks in December and January totaling $2.6 million to Endorse Liberty, a super PAC running ads on behalf of Rep. Ron Paul (Tex.), records show. Thiel, an iconoclastic former chess master who studied philosophy at Stanford University, has long backed libertarian causes and runs a San Francisco venture-capital firm.

“Men and women who want freedom and growth should take action,” Thiel said in a recent statement. “A good place to start is voting for Ron Paul.”

Another big check writer is Foster Friess, a Wyoming investor and evangelical Christian who has emerged as an enthusiastic and — for the cloistered world of top political donors — unusually talkative supporter of former senator Rick Santorum (Pa.). Friess’s volubility caused Santorum some trouble last week, when he made an ill-advised joke on television equating birth control to an aspirin held between a woman’s knees.

Records show that Friess wrote a series of checks to the Red, White and Blue Fund, a pro-Santorum super PAC: $250,000 in November, $81,000 in December, and $253,400 and $415,600 in January, after Santorum’s strong showing in the Jan. 3 Iowa caucuses.

“Don’t tell my wife how much I’m giving — she’ll kill me,” he joked in a recent interview.

Friess has been a longtime fixture in GOP political circles, giving millions of dollars to, among others, the Republican Governors Association and groups headed by the conservative Koch brothers. He said the new campaign finance rules allow him to give as much money as he wants to help a candidate who he thinks will be best positioned to beat Obama.

“I’m 100 percent behind replacing Obama, and I think Rick has the best chance of winning,” Friess said.

Priorities USA Action, a super PAC supporting Obama, has had relatively little luck attracting big-dollar donors, relying on Hollywood mogul Jeffrey Katzenberg for nearly half of the $4.4 million it has raised since last year. The president reversed himself this month by deciding to allow aides to raise money for the group.

The king of the super PACs is Simmons, 80, of Dallas, who is known as one of the creators of the leveraged buyout. He oversees a vast empire of holdings that include chemical and waste services and has an estimated net worth of $4.5 billion.

Simmons helped bankroll the controversial “Swift Boat Veterans for Truth” attacks on Sen. John F. Kerry (D-Mass.) in the 2004 presidential race. He also helped fund a $3 million attack ad in 2008 that attempted to link Obama to Weather Underground founder Bill Ayers. It was one of the biggest independent political expenditures of that cycle.

A spokeswoman said Tuesday that Simmons “has a policy of not giving interviews.”

Simmons’s contributions for 2012 have been eclectic. He directed $12 million to American Crossroads, a super PAC running ads against Obama; $1 million to help Gingrich; $1.1 million to aid Texas Gov. Rick Perry, a longtime political ally who is no longer in the race; and $100,000 to Restore Our Future, a pro-Romney super PAC.

Andrew Wheat, research director at Texans for Public Justice — a watchdog group that has criticized a radioactive-waste dump that Simmons opened in the state — said the billionaire’s super PAC contributions suggest that he will rally around the eventual GOP nominee.

“Based on the evidence, I’d say it’s sounding like an anybody-but-Obama-type effort,” Wheat said.

Staff researcher Lucy Shackelford contributed to this report.