Newt Gingrich and Mitt Romney found themselves on the defensive in the last Republican debate before the Iowa caucuses, with Gingrich challenged on whether he can defeat President Obama and Romney questioned about his consistency on social issues.

The questions highlighted the choice GOP voters will face as they start the process of selecting a presidential nominee in January: whether Romney can be trusted to lead a party that has become more conservative in recent years versus whether Gingrich has the discipline and consistency to carry the Republican banner.

Addressing doubts about his electability, Gingrich said that if the same question had been asked about Ronald Reagan in 1980, he never would have won the nomination. He cited his record in helping to enact welfare reform and balanced budgets as speaker.

“I think it’s fair to say that my commitment to disciplined, systematic work is fairly obvious,” he said. “You know, people just have to decide. . . . I strive for very large changes, and I’m prepared to really try to lead the American people to get this country back on the right track.”

Romney tried to fight back against questions about changes in position that dogged him in his 2008 campaign and remain an issue in the minds of many Republican voters. He acknowledged changing positions on abortion but said that on gay rights he has remained consistent in supporting tolerance but opposing same-sex marriage.

“I’ve learned over time, like Ronald Reagan and George H.W. Bush and others, my experience in life over 17, 18, 19 years has told me that sometimes I was wrong,” he said. “Where I was wrong, I tried to correct myself.”

Gingrich came under repeated attack, with Rep. Michele Bachmann (Minn.), who needs a strong showing in Iowa, taking a very aggressive stance against him. She challenged the former House speaker over the $1.6 million he received as a consultant to Freddie Mac and on supporting Republican candidates who favored late-term abortions.

Twice he accused her of getting her facts wrong. “Sometimes Congresswoman doesn’t get her facts very accurate,” he said the second time. Bachmann, her voice dripping with indignation, shot back: “I think it’s outrageous to continue to say over and over through the debates that I don’t have my facts right when, as a matter of fact, I do. I am a serious candidate for president of the United States and my facts are accurate.”

Thursday’s debate came in a week in which the tone of the campaign has turned more negative, with most of the attacks aimed at Gingrich and at a point when personal campaigning and television commercials in Iowa are beginning to play a larger role. The caucuses are less than three weeks away, with New Hampshire’s primary coming a week after Iowa.

There was no clear winner Thursday night and no obvious loser. No one committed a major mistake, and some of the strongest performances were turned in by candidates in the lower tier of the competition, demonstrating both the urgency of attracting more support and the improvement of virtually everyone in the field over the course of so many debates.

The debate lacked the fireworks between Romney and Gingrich that marked last Saturday’s forum in Des Moines. Romney spent the week attacking Gingrich in media interviews but did not use the debate to follow up. He generally held back from direct encounters, preferring instead to emphasize his business background and to concentrate his criticism on the president.

Gingrich made an offhand reference to a comment by Romney, who on Wednesday called the former speaker “zany,” but generally tried to keep a positive tone.

But on the issue of electability, Romney tried to draw a contrast with his rival by highlighting his background, rather than attacking Gingrich’s. He said his private-sector experience made him a better candidate given the state of the economy and the contrast between his experience and the president’s.

“I can debate President Obama based upon that understanding. And I’ll have credibility on the economy when he doesn’t. . . . This president doesn’t know how the economy works. I believe to create jobs, it helps to have created jobs.”

But if Romney held his fire, the others did not. “The speaker had his hand out and was taking $1.6 million dollars to influence senior Republicans to keep the scam going in Washington, D.C.,” Bachmann said of Gingrich. “That’s absolutely wrong. We can’t have as our nominee of the Republican Party someone who continues to stand for Freddie Mac and Fannie Mae. They need to be shut down, not built up.”

Gingrich took issue with the characterizations of his rivals. “I never lobbied under any circumstance,” he said. “ . . . I was a national figure who was doing just fine doing a whole variety of things, including writing best-selling books, making speeches, and the fact is, I only worked with people whose values I shared.”

Gingrich’s conservative credentials also came under attack. Earlier this week, Romney accused him of being “an extremely unreliable conservative.” On Thursday night, former Pennsylvania senator Rick Santorum noted that as speaker, Gingrich “had a conservative rebellion against him.”

Against those charges, Gingrich again returned to his record as speaker and his work with Reagan and the late Jack Kemp, who helped popularize supply-side tax cuts as a member of Congress. “I think on the conservative thing, it’s sort of laughable to suggest that somebody who campaigned with Ronald Reagan and with Jack Kemp and has had a 30-year record of conservatism is somehow not a conservative,” he said.

Texas Gov. Rick Perry picked up on the Freddie Mac issue by arguing that trying to make a distinction between consulting and lobbying highlighted what’s wrong with the culture in Washington.

Perry, whose debate performances have hurt his campaign, also tried to make a virtue out of the fact that he is underrated as a candidate, turning to Tim Tebow, the Denver Broncos quarterback, who was discounted coming out of college and whose skills are still heavily debated, for inspiration. “I hope I am the Tim Tebow of the Iowa caucuses,” he said.

Perry hopes that invoking Tebow, who talks openly about his Christian faith and is a hero to many social conservatives, will have particular appeal among many Iowa Republicans.

Rep. Ron Paul, who is a threat in Iowa, also found himself under attack Thursday, with Bachmann leading the charge against him over his posture toward Iran. Paul has consistently argued against U.S. intervention abroad and for ending the war in Afghanistan and raised questions about taking a provocative stance against Iran.

“It’s another Iraq coming,” Paul said. “It’s war propaganda coming. To me, the greatest danger is we will have a president who will overreact.”

At that, Bachmann said: “With all due respect to Ron Paul, I think I have never heard a more dangerous answer for American security than the one that we just heard from Ron Paul.” She added: “The reason why I would say that is because we know, without a shadow of a doubt, that Iran will take a nuclear weapon. They will use it to wipe our ally Israel off the face of the map, and they’ve stated they will use it against the United States of America.”

Romney tried to project similar strength by sharply criticizing Obama’s handling of Iran’s capture of a U.S. spy drone as overly timid. Romney said the president’s posture invites aggression, even war, on the part of America’s enemies.

“This is a president with the spy drone being brought down, he says pretty please?” Romney said. “A foreign policy based on pretty please? You got to be kidding.”

Former Utah governor Jon Huntsman Jr. sought to make a virtue out of the independent streak he has shown as a candidate, while arguing that he is reliably conservative. “I am the consistent conservative in this race,” he said. “They are coming around to find that I am not going to pander. I am not going to contort myself into a pretzel to please any audience I’m in front of. And I’m not going to sign those silly pledges.”

The debate, the 13th of the year among Republican candidates, was held at the Sioux City Convention Center and hosted by Fox News and the Iowa Republican Party.