GREENVILLE, S.C. - Challenging Newt Gingrich's claim to South Carolina, Republican presidential candidate Mitt Romney charged into the state Friday with a key endorsement from the tea party-aligned governor, a packed campaign schedule, and plans to start airing TV ads in the early primary state.

Romney's show of force was a clear signal he intends to compete aggressively in a state that stymied him in 2008 and that Gingrich has made a cornerstone of his own campaign

While Romney was planting his flag in a Gingrich stronghold, the former House speaker from neighboring Georgia spent the day off the campaign trail, with a book-signing near Washington and private family events in the capital.

On Friday and at Thursday's debate, Romney steered clear of pointed attacks on Gingrich, entering the final sprint to the Jan. 3 leadoff Iowa caucuses with an air of confidence after a week of assailing Gingrich's leadership, judgment, and temperament.

That pivot suggested the Romney camp believes that Gingrich's recent rise in polls may have leveled off and that Romney can campaign closer to his early stance as the all-but-inevitable nominee.

The New Hampshire primary follows on Jan. 10, and South Carolina's on Jan. 21. While Romney was still in Iowa on Friday, South Carolina Gov. Nikki Haley announced she was supporting him as the best Republican candidate to take on President Obama next year.

Not that Friday was all smooth sailing for Romney.

After criticizing Gingrich for more than a week as an undisciplined loose cannon likely to be savaged by Democrats, Romney opened himself to similar complaints by saying he didn't understand Medicaid until he started working in government. One of the principal avenues of criticism against Romney is that he has spent his life wealthy and out-of-touch,

"You know, I have to admit. I didn't know all the differences between these things before I got into government," Romney, referring to the federal-state health-care program for the poor, said at a campaign stop in Iowa.

The former Massachusetts governor later told reporters traveling with him to South Carolina that he understood the program but hadn't quite grasped how it was funded. He called his earlier comment a "self-deprecating understatement."

He had no harsh words for Gingrich - seeming content to leave that to his fellow Republican rivals and a political action committee that supports Romney. The closest he came Friday was a veiled reference to the former congressional leader and longtime Washington consultant.

"What concerns me is that we have in Washington, D.C., a class of people who spent their whole time in Washington," Romney said.

Haley's ties with Romney run deep. She endorsed him in 2008 when she was in the Legislature. Romney returned the favor when she ran for governor in 2010.