— Mitt Romney swooped into South Carolina on Friday to pick up the endorsement of Gov. Nikki Haley, a tea party star whose backing was intended to signal growing acceptance of Romney among conservative Republicans.

The endorsement comes at a critical time, as the surge of Newt Gingrich has forced Romney, the establishment front-runner, to navigate the most treacherous stretch of his campaign so far. Romney’s strategists believe the former House speaker’s momentum has been blunted, not only by attacks launched by Romney, but also by Thursday night’s debate that saw Gingrich come under fire from several other candidates.

Republican voters will begin picking a nominee in just 18 days, leaving little time for the hopefuls to close the sale. Rep. Michele Bachmann (Minn.) and Texas Gov. Rick Perry dashed across Iowa on bus tours Friday, while Gingrich retired to his Virginia home to prepare for the final push.

Gingrich has made South Carolina a cornerstone of his strategy and has built a double-digit lead here. But buoyed by the backing of Haley and her political organization, Romney said he hopes to challenge Gingrich for a win in the Jan. 21 primary.

Republican strategists unaligned with any campaign said Romney seemed to have stabilized his campaign this week, and for the first time showed he was fighting hard for the nomination.

The Romney campaign has “weathered a turbulent period and seem to have regained equilibrium in the context of the Gingrich surge,” said Steve Schmidt, the top strategist on John McCain’s 2008 campaign.

With Gingrich off the campaign trail, Romney tried to seize momentum with a display of organizational force. The former Massachusetts governor began airing his first television advertisement in South Carolina, and was planning a return visit to New Hampshire next week.

Romney, who has been husbanding his resources and who raised millions more in a series of large fundraisers in the New York area this week, is doubling down on his strategy to compete state by state in what could be a long quest to amass enough convention delegates to secure the nomination.

“I see every state as a battlefield,” Romney said.

He appeared to stumble Friday morning when he said in Sioux City, Iowa, that he didn’t understand Medicaid until he started working in government. Democrats seized on the remark to paint Romney as wealthy and out of touch with the poor Americans who benefit from the government health-care program.

Romney later told reporters that his comment was a “self-deprecating understatement.” He said he knew the premise of Medicaid, but did not understood the details of its structure or financing until he entered politics.

For the first time, Romney ferried reporters from Iowa to South Carolina aboard a charter plane that his wife, Ann, playfully dubbed “Hair Force One,” in honor of her husband’s slick salt-and-pepper bouffant. The candidate seemed upbeat, telling reporters stories about his father, his years in France as a missionary and his past Christmas vacations with his grandchildren.

Together with Haley, the Romneys embarked upon a two-day fly-around of the Palmetto State, beginning Friday with a late afternoon rally with several hundred supporters inside a Greenville fire station. It was perhaps Romney’s first overflow event of the campaign, with about 150 more supporters outside the packed station.

“Polls remind me a bit of going on a date, which is, you know, during polls they ask you who you’re going to vote for, you put a name out there — I’m kind of dating this person — and you go steady for a while. But before you get married, you take it real seriously,” Romney said.

With Haley looking on admiringly, Romney added: “I’m planning that if you take a close look at all the presidential contenders, give ’em a good kick in the leg, get to know who they are, that you’re going to end up supporting me for the next president of the United States.”

Haley, 39, followed an unlikely path as a daughter of Indian immigrants to become her state’s first female governor. She represents a new generation of Republican leaders, and most of the presidential candidates courted her support. That includes Romney, who was an early backer of her 2010 gubernatorial campaign.

Haley said she realized her party would not produce “a perfect candidate,” and that she looked for a candidate with “proven results” and without any ties to Washington — “the biggest chaos we know right now in this country is Washington, D.C.,” she said. Haley said she settled on Romney after he convinced her he would repeal President Obama’s health-care overhaul.

“The Governor Romney I knew in 2008 is different than the Governor Romney I met in 2011, because the Governor Romney in 2011 has had four years to think about it,” Haley said. “He’s no longer a candidate that’s trying to win. He’s already a leader that knows what he wants to do the first day he gets into office.”

Stopping in a coffee shop in Iowa, Perry, who also sought Haley’s backing, said, “I respect Nikki Haley. Good governor. She made her choice and we’ll see who’s right.”

Staff writer Sandhya Somashekhar in Iowa contributed to this report.