Texas Gov. Rick Perry sent word that he would join the race, casting a shadow over the debate Thursday night and threatening to upend the race.
Back in Iowa, Romney emerged unscathed with his leader-of-the-pack status intact after two feisty hours; his two Minnesota rivals - Rep. Michele Bachmann and former Gov. Tim Pawlenty - sparred repeatedly as each sought advantage ahead of Saturday's Iowa straw poll.
Overall, the dynamics of the campaign did not change with a single debate. And they may not change when Saturday's straw poll results are announced.
But the race could well change in the coming days as Perry dives in.
The Texan may pose the biggest threat yet to Romney.
Conservatives who make up the core of the GOP primary base view Romney skeptically on cultural issues, and he hasn't been able to establish himself as the heavy favorite for the nomination even though he's spent months promoting his background as a businessman and claiming that he alone has the know-how to create jobs to pull the country out of a period of high unemployment, rampant foreclosures and tumultuous financial markets.
Democrats are already taking aim at Perry.
"His record will get scrutinized," David Axelrod, senior political adviser to President Barack Obama, said Friday. Axelrod suggested Perry was taking too much credit for Texas' relatively healthy economy and job creation.
"He's been the beneficiary down there of the boom in oil prices and increased military spending because of the wars," Axelrod said on CBS' "Early Show." "I don't think many people would attribute it to the leadership of the governor down there."
Perry could benefit from GOP suspicion about Romney.
The Republican establishment has a lackluster view of Romney's candidacy, leading deep-pocketed donors across the country to look for more candidates to draft into the race who could bridge the historical tension between the party's social and economic wings. They couldn't convince former Florida Gov. Jeb Bush to run. New Jersey Gov. Chris Christie repeatedly refused, too. And Indiana Gov. Mitch Daniels declined overtures as well.
He is credible on issues social conservatives care about and sent a strong message to evangelicals last weekend by hosting a national prayer rally in Houston that drew roughly 30,000 Christians. He also has overseen a period of job growth in his state, making Texas one of the few states in the country that have posted economic gains and giving him the opportunity to challenge Romney's pitch as the jobs candidate.
Iowa, with its strong base of evangelical voters, may be tailor-made for Perry. He was making his first trip to the state Sunday, a day after formally announcing his candidacy in South Carolina and New Hampshire - just as Iowa straw poll votes are being cast.
A caucus campaign by the Texan could force Romney to retool his strategy of downplaying the state - which he lost during his first run in 2008 after investing heavily - in favor of friendlier ground elsewhere.
"Perry hasn't shown up in the rodeo yet, but it looks like a Romney-Perry race," Republican strategist Jim Dyke said. That may be a bit premature.
Perry is entering the race months after other candidates and Romney has a multimillion-dollar head start in fundraising.
Also, there still are at least five months before Iowa's precinct caucuses that kick off the winter-to-summer GOP nomination season, and there still are several unknowns, including whether former Alaska Gov. Sarah Palin ends up running. She was making a last-minute visit to the Iowa State Fair for Friday, reviving talk of a potential candidacy on the eve of the straw poll at Iowa State University. The straw poll could winnow the GOP field and indicate which candidate has the strongest get-out-the-vote operation.
This is turning out to be the most consequential week yet in the 2012 Republican presidential nomination fight - but not because of anything that happened at the debate.
Romney largely kept his criticism on Obama and the incumbent Democrat's handling of the economy, an issue that has blossomed anew as the GOP's top campaign concern in the wake of a tumultuous week on Wall Street and continuing high unemployment.
"I understand how the economy works," Romney said during the debate, noting the lessons of both successes and failures as a venture capital firm chief executive officer. "Our president doesn't understand how to lead or grow an economy."
He wouldn't bite when asked to comment on his rivals' economic positions.
And Romney's rivals gave him a pass on a potentially problematic comment he made earlier in the day at the Iowa State Fair when confronted by hecklers, who suggested corporations should pay more taxes. That prompted Romney to respond, "corporations are people."
Democrats quickly jumped on the exchange, though his GOP rivals did not.
Those who tried to knock him down a rung didn't even nick him.
Struggling to find traction, Pawlenty poked at Romney on several issues, including how much land he owns as well as his support for a Massachusetts health care bill similar to the national one Obama signed into law.
But Pawlenty ended up getting pulled into a family fight with Bachmann, who has outshone him in Iowa despite his 18 months of laying groundwork for a campaign.
"It's an undisputable fact that her record of accomplishment and results has been nonexistent," Pawlenty said, adding: "She's got a record of misstating and making false statements."
Bachmann, who has eclipsed Pawlenty since entering the race, quickly responded with a list of what she called Pawlenty's liberal policies when he was Minnesota's governor, including his support for legislation to curb industrial emissions and his backing of an individual health care mandate in Minnesota, both unpopular positions with GOP activists.
"You said the era of small government is over," she told Pawlenty. "That sounds a lot like Barack Obama if you ask me."
Former Utah Gov. Jon Huntsman - making his first debate appearance - also tried to claim the space as the economic-focused candidate by championing his state's job gains during his tenure and noting his time as an executive in his family's chemical company. But Obama's former ambassador to China also defended his work under the Democratic president as well as his support for civil unions - both issues that are problematic in a GOP primary campaign.
Lesser-known candidates tussled for position, including former Pennsylvania Sen. Rick Santorum, Texas Rep. Ron Paul, businessman Herman Cain and former House Speaker Newt Gingrich.
Perry was absent from the stage. But not for long.
Thomas Beaumont covers the Republican presidential race for The Associated Press.