Pablo Martinez Monsivais, Associated Press
Republican presidential candidate, former House Speaker Newt Gingrich autographs a campaign sign, Wednesday, Jan. 18, 2012, at BobbyÍs BBQ in Warrenville, S.C.

WINNSBORO, S.C. — Republican presidential candidates Newt Gingrich and Mitt Romney engaged in a harsh volley of attacks Wednesday, with Gingrich predicting an "unendingly dirty and dishonest" end to his rival's campaign and Romney mocking Gingrich's claim of helping to create millions of jobs.

Romney said Gingrich's boast was like "Al Gore taking credit for the Internet."

Gingrich said: "I fully expect the Romney campaign to be unendingly dirty and dishonest for the next four days because they are desperate."

The increasingly bitter and personal tone between the two candidates suggested that Romney sees the former House speaker a rising threat to his front-runner's status in Saturday's South Carolina primary while Gingrich is scrambling to capitalize on his momentum after a fiery debate performance Monday night.

Romney's strategy has been to leave his rivals to fight among themselves while he behaves like the nominee and focuses his rhetoric on Democratic President Barack Obama's record.

But on Wednesday, Romney shifted to Gingrich, arguing that he had overstated his role in helping President Ronald Reagan create millions of jobs.

"He'd been in Congress two years, when Ronald Reagan came to office," Romney said during an appearance at Wofford College in Spartanburg. "That would be like saying 435 congressmen were all responsible for those jobs."

Separately, supporters of Romney who served with Gingrich in Congress labeled him "erratic" and "unreliable." They also said he had helped Democratic President Bill Clinton win re-election. They spoke to reporters on a conference call arranged by Romney's campaign.

Gingrich called that "just stupid" and ticked off his record of fighting with Democratic House Speaker Tip O'Neill and engineering the first House Republican majority in 40 years with the 1994 elections.

"My only question is, so what did Mitt Romney do? Who did he help elect? What was he doing during those years?" Gingrich asked.

Later Wednesday, at a packed campaign stop at Bobby's Bar-B-Q Buffer in Warrenville, Gingrich urged more than 200 supporters to reject the better-funded and better-organized Romney.

"They thought they could buy this. They're discovering they can't buy this," he said.

Gingrich is fighting to emerge as the conservative alternative to Romney, winner of the first two contests in Iowa and New Hampshire. A third win by Romney on Saturday would make him difficult to beat in the race for the Republican nomination, and Gingrich already has conceded as much. He, former Pennsylvania Sen. Rick Santorum and Texas Gov. Rick Perry still are hoping to halt his momentum, but are splintering conservative support in the process.

Asked Wednesday if the only way for him to win would be if Perry and Santorum withdraw from the race, Gingrich replied: "It would be helpful." But he quickly added that it was not his place to tell them to get out.

"My pitch is, if conservatives come together, we beat Romney decisively. If conservatives are split, he might squeak through with a plurality. So I am trying to get every conservative voter in this state to decide, while they may like somebody else, that historically we need to get the vote for Gingrich."

On a separate issue, Gingrich told reporters that he paid 31 percent of his 2010 income in taxes, more than double the 15 percent Romney said he pays. A Gingrich spokesman said the 31 percent was the effective federal rate on his income.

Gingrich declined to criticize Romney on the tax issue, saying instead that it made the case for his own proposal to put in place an optional 15 percent flat income tax.

"My goal is not to raise Mitt Romney's taxes, but to let everyone pay Romney's rate," he said.

Gingrich said he would release his 2010 tax return on Thursday, and follow up with his 2011 return after it has been prepared.

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