"They've gone up, they've come back down," Mr Romney said of his main challengers during an appearance on MSNBC after several national polls showed that the 15-point lead held by Mr Gingrich, the former Speaker of the House of Representatives, had disappeared in barely 10 days.
Republican strategists attributed Mr Gingrich's dramatic slide to a fierce and coordinated barrage of attacks from Mr Romney, the Republican establishment favourite, who launched similar broadsides in recent months against other rivals who threatened him.
A new ABC News/Washington Post indicated that Barack Obama might be benefiting from the negative turn. The survey showed that 46 percent of voters expect Mr Obama to be re-elected, compared to 37 percent two months ago.
At the same time, Mr Obama's approval ratings, while still dismal, have crept up to their highest level for several months. Continued uncertainty in the Republican race had prompted renewed speculation that Jeb Bush, the former Florida governor, or Sarah Palin, the 2008 vice-presidential candidate, might enter the race at the 11th hour.
A confident Mr Romney, a former governor of Massachusetts, said that Mr Gingrich's lead was "not as much as it used to be but that's sort of the process that other people have gone through." He added: "I expect he'll lead till he doesn't lead."
Mr Romney has held steady in the 16 to 25 percent range of support through the year and seen rivals Michele Bachmann, Rick Perry and Herman Cain surge past him and then plummet almost as quickly. Romney advisers believe Mr Gingrich is following a similar trajectory.
A PPP poll in Iowa, the first-voting state, which goes to the polls on January 3rd, showed Mr Gingrich slumping to third place behind Ron Paul, the maverick anti-war libertarian, and Mr Romney - an ominous sign for the former Speaker.
On the stump in Iowa, Mr Gingrich hit out at the television attack ads launched by the well-funded Mr Romney and his allies.
"The next time you see one of the candidates who's running the negative ads, ask them to take it off the air," he implored some 200 people in Hiawatha, Iowa.
In Davenport, Iowa, he said that his rivals were unleashing "so much negative junk" to rip him down that they were "in effect doing Barack Obama's work" for the general election.
But Mr Gingrich has been taking aim at Mr Romney, despite his pledge t remain relentlessly positive. "Some candidates have been running for five or six years, and have raised millions and millions of dollars. And they're better organised than I am."
He also took potshots at Dr Paul, who has lambasted calls from Mr Gingrich and others for military action against Iran and decried US over-involvement abroad.
"I really stand apart from some of our candidates in believing that we need a strong defense, we need a very strong and, I think, thoroughly modernized intelligence community," said Mr Gingrich.
When asked about those comments, Gingrich denied he was talking about Dr Paul but added with a glint in his eye: "If you want to draw that conclusion, I can't stop you."
When challenged about whether he was going back on his word by attacking his rivals, Mr Gingrich responded: "Every once in a while, I slip when they get my goat and I can't quite help myself."
