McCain split registered Republicans with former Massachusetts Gov. Mitt Romney and won decisively among independents who voted in the GOP primary, an exit poll showed. But he lost among one key group: Voters who consider themselves conservatives favored Romney.
That suggests McCain faces an uphill battle in states where the Republican electorate is more conservative than in New Hampshire.
"I don't think this makes him the national leader" among Republican candidates, said Whit Ayres, a GOP pollster. "I don't think there is a national leader. I think this keeps it completely wide open."
Political analysts said the next primary, in Michigan on Tuesday, will probably be a three-way contest among McCain, Romney and former Arkansas Gov. Mike Huckabee, who won last week's caucuses in Iowa but finished a distant third in New Hampshire.
After that, the Republicans head for South Carolina, where social conservatives are likely to play a major role in voting Jan. 19.
Florida, where independents cannot vote in party primaries, casts its ballots Jan. 29; polls show former New York Mayor Rudolph W. Giuliani, who finished well back in New Hampshire, in a leading position in Florida.
California and 20 other states hold Republican primaries and caucuses on Feb. 5, choosing almost half of the party's convention delegates in a single day.
That compressed schedule, combined with the fragmentation of support for the leading candidates, means that McCain faces determined competition from Romney, Huckabee and Giuliani.
In his concession speech in Bedford, N.H., Romney said he was determined to "fight across this nation, on to Michigan and South Carolina and Florida and Nevada and states after that."
Huckabee said his third-place finish gave him "continued momentum."
And Giuliani told supporters: "Think of it as the kickoff in what's going to be a very long and very tough game."
McCain's strong showing among registered Republicans -- as measured in the exit poll sponsored by a news media consortium -- surprised many analysts, who expected the Arizona senator to rely on independent voters.
But as Neil Newhouse, a Republican pollster, noted, "He's won this one -- but where's his second victory going to come?"
McCain's win gives him a jolt of momentum as he heads for Michigan, where the GOP electorate is similar to New Hampshire's: relatively moderate and open to independents.
McCain won the state in 2000, although -- as he confessed this week on his campaign bus in a characteristic burst of candor -- that victory was "one that always puzzled me."
Also working in McCain's favor: Barack Obama and other major Democratic candidates are bypassing their party's primary in Michigan, so McCain will face no competition with the Illinois senator for independents, as he did in New Hampshire. Recent polls have shown that the state is a tossup.
Romney has counted on his family ties in Michigan -- his father, George, was the state's governor in the 1960s -- although one of his advisors said that might count only among voters who were "a little long in the tooth."
"Romney now really has to win in Michigan -- it's really his do-or-die state," said Scott Reed, who managed Bob Dole's unsuccessful Republican presidential campaign in 1996.