WASHINGTON — As Republican leaders watch with horror Newt Gingrich’s one-man campaign to bring down the party’s likely presidential nominee, they should remind themselves of this: Gingrich is a monster of their own making.

The former House speaker has almost no public support. In New Hampshire, 90.6 percent of Republican primary voters cast their ballots for somebody else. In Iowa, 86.7 percent of caucus-goers chose somebody else. In the two states combined, Gingrich received fewer than 40,000 votes — a tally that wouldn’t even land him on some city councils.

But there are 5 million reasons Republicans have to fear Gingrich. That’s the number of dollars billionaire casino magnate Sheldon Adelson gave to a pro-Gingrich super PAC — the largest contribution to a candidate’s cause in U.S. history. This allowed the Winning Our Future PAC to buy $3.4 million worth of ads in South Carolina — enough to saturate the state with poisonous messages about Mitt Romney. The almost half-hour video taking apart Romney’s performance at Bain Capital, released Wednesday, provides a taste of what’s to come in the next week.

Liberals have complained for two years about the Supreme Court’s Citizens United decision, which made such unlimited contributions possible. But Republicans on Capitol Hill resisted attempts to limit the damage of the decision — and now Gingrich is teaching them the consequences of their own actions.

Having fully recovered from his pledge last month to run a “relentlessly positive” campaign, he arrived in South Carolina on Wednesday with renewed defiance of those Republicans who have called on him to soften his attacks on Romney.

“I think that the American people deserve to know things,” he told reporters in Rock Hill, S.C. “I’m prepared to have people be irritated on the right and the left.”

But people on the left are delighted with Gingrich just now. He and his allies are making the case against the likely Republican nominee better than President Obama and the Democrats ever could.

“Consider Mitt Romney,” says a new Gingrich web video, set to clown music and featuring a highlight reel of Romney’s verbal gaffes: his claim that “I like being able to fire people,” his argument that “corporations are people,” his explanations for the lawn company he hired that used illegal immigrants, his $10,000 wager with Rick Perry, his fears about getting a “pink slip,” his claims of hunting for “small varmints,” his belief that his dog enjoyed riding in a kennel strapped to the top of the Romneys’ car, even his awkward attempt to sing “Who let the dogs out?”

Rick Perry has joined in with attacks on Romney’s “vulture capitalism.” But Perry’s words don’t matter much because he doesn’t have a fresh $5 million contribution to devote to disemboweling Romney. Gingrich was more likely the one Romney had in mind when he complained Tuesday about the “bitter politics of envy.” Romney, on his way to South Carolina, complained explicitly that Gingrich was against “free enterprise.”

Romney has it wrong. Gingrich’s attacks on him are the very essence of free enterprise: They’re helped by campaign finance laws that sell elections to the highest bidder. For those Republicans who thought that unlimited political contributions would be a good thing for their party, it’s a delicious irony that a casino billionaire is using his money to underwrite a populist assault on the GOP front-runner.

“Crony capitalism, where people pay each other off at the expense of the rest of this country, is not free enterprise, and raising questions about that is not wrong,” Gingrich said in South Carolina. Americans, he said, should know whether businesses are “fair to the American people, or are the deals being cut on behalf of Wall Street institutions and very rich people.’’

If Republican elites don’t like millions of dollars being spent to amplify that anti-Romney message, they have only themselves to blame.