After preparing for a drawn-out nominating battle that would stretch well into the spring, Mitt Romney’s presidential campaign is now quietly shifting gears in an effort to steamroll his underfunded opponents — and lock up the Republican nomination by the Florida primary at the end of this month.

Buoyed by a narrow win in the Iowa caucuses and his commanding lead in the New Hampshire polls, Romney has turned his attention to South Carolina, where he is dispatching a slew of high-profile surrogates and relocating some staffers ahead of the Jan. 21 primary. Looking further ahead, Romney has begun a massive advertising blitz in Florida and launched an aggressive outreach program to early voters in the state.

Romney campaign advisers insist that they are moving forward one state at a time and not taking any contest for granted. Yet Republican observers see Romney executing an ambitious strategy that would quickly maximize his momentum and try to quash any further surges by his rivals.

“If Romney wins the first four states, he’ll be the de facto nominee of the party,” said Steve Schmidt, a senior strategist on Sen. John McCain’s 2008 GOP campaign who is unaffiliated in the current race. Ed Rogers, another unaffiliated Republican strategist, said the notion that Romney may wrap up the nomination by Jan. 31 is “perfectly plausible.”

The new Romney push hinges on his performance in South Carolina, a more traditionally conservative state where he finished fourth in 2008 and until recently appeared to face an uphill fight. But South Carolina’s conservatives do not yet appear to be uniting around a single alternative, opening a path to victory by plurality. A poll released Friday showed Romney with an 18-point lead in the state over his closest opponent, former Pennsylvania senator Rick Santorum, who had 19 percent support; former House speaker Newt Gingrich followed at 18 percent.

Romney is airing television advertisements across South Carolina, the most recent highlighting his opposition to the National Labor Relations Board’s unsuccessful attempts to prevent Boeing from moving its operations to the state to avoid dealing with unionized workers elsewhere. On Thursday afternoon, Romney swooped into Charleston and Conway for an 18-hour campaign swing with McCain, who won the South Carolina primary in 2008 on his way to the nomination. McCain has been calling his supporters there urging them to get behind Romney.

“He’s gonna win in New Hampshire, and it’s gonna come down, my friends, as it always does, to South Carolina,” McCain (Ariz.) said Friday.

Other surrogates, including New Jersey Gov. Chris Christie, will visit South Carolina before the Jan. 21 primary, while some of Romney’s Iowa staff members have begun relocating there. Romney plans to return next week. “As soon as New Hampshire’s over, he’ll be living here,” said South Carolina Treasurer Curtis Loftis, Romney’s state chairman.

Romney is also reaching out to tea party activists with an electability message. Loftis addressed a tea party rally in Greenville this week, while South Carolina Gov. Nikki Haley is mobilizing the grass-roots network she assembled for her 2010 race. Loftis said he cut a radio advertisement on Romney’s behalf, along with Haley and conservative commentator Ann Coulter.

Now, Romney is courting the endorsement of Sen. Jim DeMint (S.C.), a potential tea party kingmaker who had backed him in 2008. DeMint’s relationship with Santorum, who finished second to Romney in the Iowa caucuses this week, is said by confidants to have soured considerably since they served together in the Senate.

“I’m sure the campaign will scold me for setting any kind of expectations, but I do think Governor Romney will do very well,” Loftis said. “People are calling me saying, ‘Curtis, I didn’t get it in August, but I get it now.’ People want to send Barack Obama back to Chicago, and while they like Newt Gingrich and Rick Perry and Rick Santorum, when they get into that ballot box they’ll say, ‘Do I want to finish in first place or last place?’ ”

But before the contest in South Carolina, Romney must at least match, if not exceed, the soaring expectations for his performance in New Hampshire on Tuesday.

As a result, Romney returned to New Hampshire on Friday for a spaghetti dinner with Haley in tow. A “grass-roots rally” is scheduled Saturday morning ahead of the Republican debate that night. Another debate and six more events in New Hampshire are planned before Tuesday.

“We take everything in order, one state at a time,” senior Romney adviser Eric Fehrnstrom said. “But we have the ability to walk and chew gum at the same time.”

Some see Romney’s aggressive push as a defensive strategy and said the possibility that Romney could lose South Carolina has only reinforced his need to play simultaneously in the other contests this month. “Romney has to build firewalls everywhere he can because South Carolina is the place where he could really be damaged,” said veteran Bush strategist Mark McKinnon. “For 30 years, South Carolina has picked the winner, and they generally don’t much like Yankee elites like Romney. He may run out of votes in South Carolina, so the key is not to run out of money and organization in the states that follow.”

If Romney triumphs in South Carolina, said Rogers, the Republican strategist, “then he would really control his destiny.”

“If he wins South Carolina, it’s hard to see who and where somebody actually overwhelms his campaign,” Rogers said.

In Florida, a much larger state where expensive television advertising is critical, Romney began a month-long ad blitz this week. His campaign has also contacted, at least twice, the more than 450,000 Floridians who have requested absentee ballots. Romney will be in Florida for a Jan. 12 fundraiser in Palm Beach that a campaign official said has nearly 100 co-hosts and is expected to bring in more than $1 million.

At the national level, Romney is adding staff to help with the January push. Kevin Madden, who served as national press secretary on the 2008 campaign and has given informal advice since, officially came on board Sunday as an adviser. Madden, a frequent television analyst who has deep ties to party leaders, will sometimes travel with Romney, counseling him on messaging, debate preparation and early-state strategy.

At the same time, Romney advisers are already plotting for a general election against President Obama. Romney’s fundraising chief, Spencer Zwick, has summoned more than a dozen national finance chairs to the campaign’s Boston headquarters Wednesday to lay plans to swiftly raise millions of dollars from new donors once Romney becomes a presumptive nominee, according to a top fundraiser.

Romney’s top aides said privately that they see a four-state streak within reach but publicly dismissed any suggestion of a coronation. They said the campaign is focused methodically on amassing the roughly 1,150 delegates required for the nomination.

“If ifs and buts were beers and nuts, we’d be having one heck of a party,” Madden said. “Our campaign wants to compete in South Carolina, wants to do well in South Carolina, but doesn’t have a formula that doesn’t add up if we don’t win it. We go on to Florida.”

In fact, Romney’s campaign is looking far beyond Florida, preparing for early February caucuses in Nevada, Colorado and Maine, the Feb. 28 primaries in Arizona and Michigan, followed by Super Tuesday contests in more than 10 states March 6. It has staff on the ground in several of those states.

But Charlie Black, a veteran operative who was a top McCain adviser and is supporting Romney, said if Romney wins the next three contests, “it’d be over for all practical purposes.”

Staff writer Rachel Weiner in Conway, S.C., contributed to this report.