Asked after a in Detroit Friday if he was the best candidate to defeat the president, Romney said, "I don't think if I have the best chance, I think I have the only chance."
Romney then retreated from the comment, saying, "Maybe I'm overstating that a bit."
But later, he said the way to beat Mr. Obama is to nominate someone who has not spent their career in Washington - adding that his rivals for the GOP nomination have done exactly that.
"I think the only way to defeat him is to have someone who runs against him who is very different than he, who can present a clear contrast," he said. "I have not spent my life in Washington."
Romney went on to say that Newt Gingrich and Rick Santorum are "fine fellows, but they spent their careers in Washington. They have as their background the same background [Obama] has."
He added that he has "credibility" on the economy because of his time in the private sector.
"I believe the best way and the only way we really have to get President Obama out of the White House and get someone there who will put Americans back to work with more jobs, less debt and a smaller government is if we nominate someone whose career is not politics, whose lived in the private sector and whose passion is caring for the coming generations," said Romney.
The comments come as the Romney and Santorum campaigns take harsh shots at each other over their economic proposals. In dueling releases Friday, Romney's campaign said Santorum's proposal reflects a "DEFICIT-DOUBLING, WINNER-PICKING, BIG-GOVERNMENT" outlook, while Santorum's camp said Romney's plan reflects ""TAXACHUSETTS ECONOMIC POLICIES."
The two campaigns also traded nasty rhetoric in the wake of Wednesday night's GOP presidential debate, when Santorum discussed his decision to cast votes he did not agree with to help his party while in the Senate.
"Mitt Romney has criticized me for taking one for the Republican team and we all know why, because Mitt Romney's teammates are all Democrats," Santorum said in a release. "It's pretty clear what team Mitt Romney is on when he passed socialized medicine that included $50 dollar abortions - bragged about not lining up with the NRA - appointed liberal activist judges to the Massachusetts bench, and was hanging out at Planned Parenthood events celebrating the pro-choice agenda. No wonder working with a Republican President's team is foreign to Mitt Romney - Mitt's whole career he's been working for liberals."
Shot back Gail Gitcho, Romney's communications director: "Rick Santorum is a Washington insider who is lashing out at Mitt Romney because he had a terrible debate performance. Back in 2008, Sen. Santorum endorsed Mitt Romney for president because of Mitt's 'conservative' record. Now, Rick's changed his tune. This sounds like another case of Rick Santorum abandoning his principles for his own political advantage."