Republican debate: Romney and Gingrich defend positions
Republican presidential front-runners Newt Gingrich and Mitt Romney were forced to defend their records, in the final TV debate before primary season.
Mr Gingrich was challenged over his work with federal housing firm Freddie Mac, while Mr Romney had to explain his stances on gay marriage and abortion.
The two rivals played it safe, but their conservative credentials were assailed by lower-tier candidates.
The eventual nominee faces Democratic President Barack Obama next November.
Thursday night's two-hour forum, hosted by Fox News in Sioux City, Iowa, also featured Texas Republican Ron Paul, Minnesota Congresswoman Michele Bachmann, Texas Governor Rick Perry, former Utah Governor Jon Huntsman and former Pennsylvania Senator Rick Santorum.
In his opening remarks, Mr Gingrich challenged the president to series of lengthy policy debates: "I believe I can debate Barack Obama and I think in seven three-hour debates, Barack Obama will not have a leg to stand on in trying to defend a record that is terrible and an ideology that is radical."
Mr Romney lambasted President Obama for trying to "appease or accommodate the tyrants of the world", criticising his approach to retrieving a drone which recently went down in Iran.
"Foreign policy based on pretty please? You have to be kidding," Mr Romney said.
He chose not to take the bait when the moderator asked whether he would like to respond to a previous challenge by Mr Gingrich that the former governor of Massachusetts should give back the millions he earned bankrupting companies while working at a private equity group.
"I think the president will level the same attack," Mr Romney said. "In the real world that the president has not lived in... not every business succeeds."
While the two front-runners sought to stay positive, despite increasingly barbed attacks in their day-to-day campaigns, they came under fire from Mrs Bachmann and Mr Santorum.
Mrs Bachmann forcefully assailed Mr Gingrich for collecting "influence-peddling" fees from government-owned mortgage lender Freddie Mac, which is blamed by many conservatives for America's home foreclosure crisis. She said his role had been to "keep the scam going".
"What she just said is factually not true," Mr Gingrich shot back. "I never lobbied under any circumstances."
Mr Santorum attacked Mr Romney, saying he handed out gay marriage licences as governor of Massachusetts.
But Mr Romney said the Massachusetts Supreme Court had ruled that gay marriage was legal, and that he had simply abided by that decision, while trying to have it overturned.
Mr Romney also acknowledged he used to favour abortion rights, but said he changed his mind as governor when he faced a bill that would have allowed experimentation on embryos.
After being given up for politically dead earlier this year, Mr Gingrich surged to the front of the pack last month as early contenders Rick Perry and Herman Cain saw their support collapse amid gaffes and sex scandals.
While Mr Gingrich retains the lead nationally, he fell to second place in Iowa in a Rasmussen Report opinion poll on Thursday.
In the survey, Mr Romney won the support of 23% of voters, while Mr Gingrich was on 20%, two points ahead of Ron Paul.
On 3 January, Iowa will hold the first in a series of state-by-state non-binding votes, known as primaries and caucuses, to pick a Republican nominee. The eventual candidate will be officially declared at the national party convention next August.
A strong showing in Iowa would provide momentum going into New Hampshire, which votes on 10 January, and the more populous South Carolina and Florida, which vote on 21 January and 31 January, respectively.