COLUMBIA, S.C. - Fresh off his primary night win in Manchester, N.H., Mitt Romney told press aboard a flight to South Carolina that he knows a win in the Palmetto State will be "an uphill battle."

"Clearly I face more of an uphill battle in South Carolina than I have in New Hampshire," said Romney, before a charter flight took off from Bedford, Mass., to Columbia, S.C., on Wednesday morning. "And with regards to South Carolina, last time I came in fourth, so our team recognizes that this is going to be a challenge."

Romney, though, was 18 points ahead of his closest rival, Rick Santorum, and 17 points up on Newt Gingrich in a CNN/Time poll taken Jan. 4-5 in South Carolina.

Romney is hitting the ground running in South Carolina, with events scheduled through the weekend across the state. But when asked whether he expected a so-called "whisper campaign" to meet him, Romney didn't rule it out.

"You know, as was said long, long ago, politics ain't beanbags," said Romney. "And I know it's gonna get tough and no one's gonna be happy if things are said that are untrue."

"But I know that's sometimes part of the underbelly of politics," said Romney. "I hope no one associated with any of my effort, whether it's my own campaign or anyone else supporting me, I hope none of them do anything that is, that departs from the truth."

Romney said that he was "a little surprised" to see Gingrich be the "first witness for the prosecution" on his career at Bain, but said he doesn't feel threatened, adding, "I don't think that it's going to hurt my efforts."

"Frankly, if I can't take a few shots coming from my colleagues on the Republican side, I'm not ready for Barack Obama," he said.

And speaking of the president, Romney opened a new line of attack on Obama, referring to him for the first time as a "venture capitalist" in his work with Solyndra, which went bankrupt after the government loaned $535 million to the energy company.

"He's been a private equity guy in General Motors and Chrysler, so I'm going to talk about his record when I'm facing him," said Romney.