Seven candidates — Pawlenty, Bachmann, former Utah Gov. Jon Huntsman, former House Speaker Newt Gingrich of Georgia, Texas Rep. Ron Paul, former Pennsylvania Sen. Rick Santorum and businessman Herman Cain — sought to separate themselves from the packed field and emerge as the chief alternative to Romney.
Pawlenty, who hesitated in a June debate to criticize the former Massachusetts governor, poked at Romney and Obama at the same time.
"Where's Barack Obama on these issues. You can't find his plans on the most pressing issues in this country," Pawlenty said, promising audience members and TV viewers he would "come to your house and cook you dinner" if they could find Obama's proposals. "Or if you prefer I'll come to your house and mow your lawn ... In case Mitt wins, I'd limit it to one acre."
Romney, who has several homes and was looking to protect his leads in national and state polls, smiled and took a pass when given a chance to respond, saying: "That's just fine."
He kept his focus on Obama, saying: "Our president simply doesn't understand how to lead and how to grow the economy." He also criticized Democrat Obama on the downgrade of the nation's credit rating.
Appearing in his first presidential debate, Huntsman acknowledged he had not yet presented an economic plan, but he cited his economic record as governor of Utah as evidence of what he would accomplish as president. He defended his service as ambassador to China under Obama as a patriotic act.
Huntsman, who is not competing in the Iowa caucuses where social conservatives dominate, also tried to differentiate himself from the rest of the field. He defended his support for civil unions and offered no apologies for other moderate positions he holds.
Gingrich, pressed on the implosion of his campaign amid financial strife and infighting earlier this summer, chastised the Fox News panel for "gotcha questions." He said Republicans including Ronald Reagan and John McCain had staff defections during their campaigns, and he said he intended, in his words, to "run on ideas."
Roughly 45 minutes into the debate, Santorum raised his hand and said: "I haven't gotten to say a lot."
Showing the wide diversity of opinion, Paul gave a staunchly libertarian answer to nearly every question from the economy to foreign affairs, essentially saying the United States should have friendly relations even with countries that violate human rights and not interfere in their internal affairs. "It's about time we talk to Cuba," Paul said at one point. He also said the United States had created the hostile relations between itself and Iran.
Even before the debate began, it was a campaign day to remember.
At an appearance early in the day, Romney was badgered by hecklers at the state fair. In response to chanting about corporations, he said that "corporations are people," a comment Democrats predicted would be a defining moment of his campaign.
Romney, who has struggled with an aloof and elitist image as he tries for the GOP presidential nomination a second time, made the remark while outlining options for reducing the federal deficit and overhauling entitlement programs.
Despite tea party outrage that sometimes focuses on banks and auto companies, Romney has said to applause from GOP audiences that the rights of business are being trampled under Obama to the detriment of the struggling economy. But in Thursday's audience, the line encountered resistance.
A few hours after Romney's awkward moment, Perry spokesman Mark Miner confirmed that the Texas governor would announce that he was running for president while in early primary states on Saturday.
Perry's candidacy is certain to upend the race, and he could challenge Romney for the role of jobs-focused candidate.
The conservative governor is seen as a potential bridge between the party's social and economic wings.
Asked about Perry's candidacy during the debate, several of his opponents welcomed him to the race — and used the opportunity to criticize him. Cain called Perry "one more politician," while Paul said he was pleased Perry was joining the field because "he represents the status quo."
- NORAD 'tracks' Santa's journey across globe
- Officer kills armed 18-year-old near Ferguson
- Women excised from public life, abused by IS
- Severe storms slam the South, killing at least 4
- Wal-Mart tests gift card exchange
- Sony broadly releases 'The Interview' in...
- Key developments in the case of slain NYPD...
- Denver shelters cite legal pot in homeless...
- Police boss: NYC cops 'quite simply,... 17
- NYPD: Cop ambush killer told passers-by... 15
- Going it alone, Obama rocked the boat... 13
- Key developments in the case of slain... 9
- Pope in blistering critique of Vatican... 8
- Officer kills armed 18-year-old near... 8
- Killings of 2 New York officers trigger... 6
- Senate, CIA agree torture program was... 6