COLUMBIA, S.C. — Republican presidential candidate Mitt Romney said Wednesday he has "an uphill climb" to win South Carolina's presidential primary but is ready to defend himself from the "underbelly" of politics in a state known for bare-knuckled tactics.

Boarding a plane for the flight to Columbia, S.C., to campaign for the Jan. 21 primary, Romney said he's prepared for the direct and indirect attacks that are sure to come from Newt Gingrich and other rivals for the GOP nomination.

"Politics ain't bean bags and I know it's going to get tough and no one's going to be happy if things are said that are untrue. But I know that is sometimes part of the underbelly of politics," the former Massachusetts governor said when asked if he was prepared for a whisper campaign about his Mormon religion or other aspects of his background.

"I hope no one associated with any of my effort, whether it's my own campaign or anyone else that's supporting me, I hope none of them do anything that departs from the truth," he said.

Romney said he was surprised that Gingrich was so aggressively attacking his past tenure at the private equity firm Bain Capital. Gingrich, soundly defeated in the first two Republican contests, has gone after the front-running Romney as a former venture capitalist who earned millions through buyouts of companies that ultimately cost people their jobs.

"We understood for a long time that the Obama people would come after free enterprise. (I'm) a little surprised to see Newt Gingrich as the first witness for the prosecution but I don't think that's going to hurt my efforts," Romney said. "Frankly, if I can't take a few shots coming from my colleagues on the Republican side I'm not ready for Barack Obama."

Romney also previewed a new line of attack against President Obama, whose campaign is certain to level similar charges against the former governor if they end up opposing each other in the general election.

Romney said Obama has been "a venture capitalist at Solyndra," referring to a California energy company that declared bankruptcy and laid off 1,100 workers despite a $528 million loan from the Obama administration. He also said Obama was a "private equity guy at General Motors and Chrysler," referring to Obama's intervention to save the companies from collapse.

"So I'll be talking about his record when I'm facing him," Romney said.

The GOP front-runner, Romney said he faces "more of an uphill battle" in South Carolina, where he is less well known than in New Hampshire, which handed him a victory in its primary Tuesday night.

When he sought the nomination in 2008, Romney finished fourth in South Carolina, spurned by religious and socially conservative voters wary of his Mormon faith and past support for abortion rights.

"Our team recognizes this is going to be a challenge," Romney said.

Earlier Wednesday, interviewed on ABC's "Good Morning America," he said: "I don't know if we can win South Carolina."

With an eye on Florida's contest at month's end, Romney began airing a new Spanish-language TV ad in that state.

Speaking with reporters traveling with him, Romney still savored his New Hampshire win with 39 percent of the vote. The victory made him the first Republican to sweep the first two contests in competitive races since 1976.

"It was like Christmas Day," Romney said. "Each new report of votes coming in was like opening another present."