NASHUA, N.H. — Senator McCain’s comeback victory here reshapes the Republican race into a wide-open fray between him and the victor in Iowa, Governor Huckabee.

It makes Michigan a must-win for Governor Romney, who finished second here and in Iowa. It will strain Mayor Giuliani’s novel strategy of focusing on Florida and the February 5 states.

The focus of the campaign now shifts to Michigan, which holds its primary January 15, and South Carolina, whose contest takes
place January 19.

The chairman of Michigan’s Republican Party, Saulius Anuzis, told The New York Sun he expected Messrs. McCain, Romney, and Huckabee to compete in his state, but not Mr. Giuliani. “The only campaign I haven’t heard from was Giuliani’s,” Mr. Anuzis said, noting that the mayor had done well in early polls. Michigan is an important swing state in general elections almost evenly split between Republicans, Democrats, and independents.

Mr. McCain received 37% of the vote, in comparison to 32% for Mr. Romney. Mr. Huckabee garnered 11%, and Mr. Giuliani beat Ron Paul by one percentage point, 9% to 8%. Senator Thompson captured just 1% of the vote.

Mr. Giuliani constructed his strategy to meet the unique challenges of this year’s front-loaded primary contest.

“It’s a whole new ball game,” a former governor of Massachusetts who is supporting Mr. Giuliani, Paul Cellucci, said. “I do believe when we wake up on February 6, he’s going to be ahead on delegates.” Mr. Cellucci argued that for the February 5 states to be “relevant” they must take their own look at candidates without regard to past performance.

“What I can tell you is this race is a wide open race,” Mr. Giuliani said last night. “There’s going to be a lot more ups and downs.” He also tried to describe the defeat as part of a broader plan: “It fits in to how we looked at this from the very beginning.”

The derision with which the plan is being greeted in some quarters — “You’re going to go 0 for 4 and win the thing?” a former Republican presidential candidate, Patrick Buchanan, asked — reflects the rapidly changing landscape of the campaign.

As recently as six weeks ago, Mr. Giuliani viewed his main opponent as being Mr. Romney. The objective was to stop Mr. Romney from winning both Iowa and New Hampshire. Since that time, Mr. Huckabee, a former governor of Arkansas, won the Iowa caucuses, and the Giuliani camp had to contend with Mr. McCain, who opted to center his attention on New Hampshire, which he won in 2000.

Mr. McCain’s supporters last night erupted into chants of “Mac is back!” when news broke that he had won the primary. The candidate took the stage shortly after 9 p.m. following the playing of the theme from the movie “Rocky.”

“Tonight we showed them what a comeback looks like,” he said. “We celebrate a victory and leave for Michigan to win another tomorrow.”

Mr. McCain’s campaign said the victory would boost its finances, another factor that hindered him the last time he ran. “We have the resources and money we need in Michigan and South Carolina,” an adviser to Mr. McCain, Steve Schmidt, said. Mr. McCain goes to South Carolina with more advantages than he did in 2000, when his presidential candidacy came to an end. First is momentum. “The momentum John has gained, going from where he was to winning, is one of the great stories of American politics,” Senator Graham of South Carolina said.

Mr. McCain also possesses more support among Christian conservatives and the Republican establishment, both of which he lacked when he lost to President Bush.

In addition, Mr. McCain’s support for the troop surge that has helped stabilize Iraq will likely be an asset for the Arizona senator in a state with a high percentage of military veterans, for whom the question of who can best serve the country as commander-in-chief is an important issue. South Carolina, Mr. Graham said, also has an elected head of its National Guard, who has endorsed Mr. McCain, giving him additional political troops in the fight for South Carolina’s voters.

“John uses the words ‘win’ and ‘victory,'” Mr. Graham, who described Mr. McCain as a Reaganite figure who could weave the Republican coalition back together, said. “John is the soldier’s candidate.”

Even so, he will face Mr. Huckabee, who won Iowa with the support of evangelical Christians, in heavily religious South Carolina. He will also face Mr. Thompson, whose campaign released an optimistic statement about his ability to perform in South Carolina, “where voters are far more conservative than in New Hampshire and deeply concerned about illegal immigration.”

Earlier in the day, Mr. Romney, appearing at a polling place in Derry, said even if he came in second place, he would have won the largest number of Republican votes in the race, based on his combined tally in Iowa, New Hampshire, and Wyoming, where he was the only candidate to compete.

Mr. Romney trumpeted his ability to perform well in the state of his birth, Michigan, where his father was governor, and where, Mr. Anuzis said, he had built a strong organization.