Aides said the Gingrich campaign had purchased a full-page newspaper ad in New Hampshire for Wednesday morning calling Romney a "Timid Massachusetts Moderate."
Romney, who finished second in Iowa in 2008 despite a costly effort, initially campaigned cautiously this time around.
But he barnstormed extensively across the state in the race's final days in pursuit of a first-place finish, running as a conservative businessman with the skills to fix the economy and as the challenger with the best chance to defeat Obama.
Santorum, Gingrich, Perry and Bachmann argued that Romney wasn't nearly conservative enough on the economy and social issues such as abortion. They vied for months to emerge as the alternative to the former Massachusetts governor.
Paul's libertarian-leaning views set him apart, and he hoped that might be enough to claim victory in a six-way race where no one broke away from the pack.
Unlike in a primary, in which voting occurs over hours, the 809 Iowa caucuses were meetings in which Republicans gathered for an evening of politics. Each presidential candidate was entitled to have a supporter deliver a speech on his or her behalf before straw ballots were taken.
Under party rules, caucus results have no control over the allocation of Iowa's 25 delegates to the Republican National Convention. The Associated Press uses the caucus outcome to calculate the number each candidate would win if his support remained unchanged in the pre-convention months.
The race in Iowa came to be defined by its unpredictability as the months rolled by and nationally televised candidate debates piled up.
Bachmann gained early momentum on the strength of a victory in a summertime straw poll and a feisty debate performance.
But she quickly faltered when Perry joined the race and overshadowed her as the 10-year governor of Texas with deep-pocketed supporters and an unbroken record of electoral success at home.
Perry's rise lasted only as long as a couple of debates including one where he memorably was unable to recall the third of three federal agencies he wanted to abolish.
Next up was Herman Cain, a black former businessman who improbably shot to the top of the polls in a party that draws its support chiefly from white voters. He suspended his candidacy a few weeks later, after a woman said she and he had carried on a long-term extra-marital affair.
Gingrich rode the next surge in the polls, a remarkable comeback for a man whose campaign had imploded earlier in 2011 when most of his aides quit in frustration. But his rise lasted only until a super PAC that supports Romney began attacking him on television.
Democrats watched carefully in a state that has swung between the two parties in recent presidential elections.
It was Iowa that launched Obama on the way to the White House four years ago when he won a convincing victory in the caucuses.
David Espo reported from Washington.
- Here's a quiz to test your Star Wars...
- LA man builds small home for homeless lady
- Report highlights earth-shattering increases...
- Britain's new princess can't expect...
- Have you seen 'Age of Ultron'? Here are 50...
- 'Avengers' sequel is second biggest US opener...
- Vonn says relationship with Woods is over
- Mayweather takes his $100 million check and runs
- Vermont Sen. Bernie Sanders: 'I am... 41
- Rage to relief in Baltimore as 6... 40
- Defending the Faith: Going up to Jerusalem 30
- Former tech executive Carly Fiorina... 17
- Governor, legislators leave 'baggage'... 14
- More Americans spending at least half... 13
- Famed neurosurgeon Ben Carson announces... 10
- Latest on police-custody death: 53... 9