If McCain can defeat Romney in Michigan, that could effectively spell the end for Romney despite his enormous war chest.
The primary came just five days after the surprise shakeups in both parties following the Iowa caucus, in which former Govs. Mike Huckabee and Mitt Romney placed first and second, respectively.
"If I come in a second-place finish, that will actually say that I am clearly one of the leading contenders," Romney told the Concord Monitor Monday. "I will have come in second in Iowa, first in Wyoming, second in New Hampshire. That will mean that I probably have more votes than anybody else in those first three states."
The first tabulated votes of the day foretold big things for McCain, giving credence to his recently unveiled slogan "The Mac Is Back." Votes were tallied at the state's first and smallest polling station in the town of Dixville Notch, with 17 registered voters, just after midnight Tuesday.
Republican voters in Dixville Notch gave the majority of their votes, all four of them, to McCain.
"And after a landslide victory last night in Dixville Notch -- 4 to 2 [for Romney] -- there's no way you can stop this momentum," he said.
At least one candidate who had no realistic shot of a strong New Hampshire finish chose to spend the day in South Carolina instead, where the GOP primary will be held Jan. 19. Former Sen. Fred Thompson of Tennessee told a crowd in Greenville, S.C., that he wasn't competing in New Hampshire.
At a hotel in Manchester, N.H., Rudy Giuliani congratulated the winner and called the race "wide opened" and said he was looking forward to competing in populous states like Florida, New York, New Jersey and California. After his concession, the former New York City mayor boarded a plane for Florida.
Rick Klein, Karen Travers, John Berman, Ron Claiborne, Bret Hovell and Gary Langer contributed to this report.