No clear front-runner has emerged after the first four state tests, and at least four Republicans have a credible chance to become the nominee.
In the Democratic primary, Sen. Hillary Rodham Clinton finished ahead of "uncommitted" but the vote had no bearing on the delegate count. Michigan was not among the four states authorized to hold Democratic primaries before Feb. 5, and Barack Obama and John Edwards removed their names from the ballot.
The Republican primary, however, was anything but meaningless. Romney was facing possible elimination as a serious 2008 contender after his well-financed campaign suffered defeats in Iowa and New Hampshire.
Instead, he revived his prospects and inflicted a severe defeat on McCain, who might find it increasingly difficult to recover.
Romney's victory puts him "back in the thick of this race, as the delegate leader and the guy with the most money," said Whit Ayres, a Republican pollster who is not involved in the presidential campaign.
McCain, who was second, remains "a very credible contender," he said. The Michigan results increase the chances that Mike Huckabee, who ran third, will win South Carolina, where Christian conservatives are a powerful force, he said.
Rudolph W. Giuliani, a distant sixth in Michigan, was a winner, nonetheless. He has retreated to Florida, after lackluster performances in the early states, and McCain's setback kept the former New York mayor's chances alive.
Romney rolled up large margins in the Detroit suburbs as ice and snow kept turnout below record levels. Election Day interviews with voters leaving their polling places showed that Romney carried self-identified Republicans by a margin of 40 percent to 26 percent and conservatives by even more, troublesome signs for McCain, whose campaign is low on money.
McCain had re-emerged as the favorite in national polling in recent days, but he's the underdog in the next two tests, in Nevada and South Carolina on Saturday.
Romney appealed to voters in his native Michigan to save his candidacy and stressed his early years in the state where he met his wife and his father was a popular governor. He made an aggressive pitch on behalf of the state's battered auto industry, criticizing a new federal law mandating more energy-efficient vehicles, and he promised to convene a summit meeting on behalf of the industry during the first 100 days of his presidency.
"Tonight marks the beginning of a comeback. A comeback for America," Romney told supporters in Southfield, Mich.
In a slap at McCain, Romney said the vote was "a victory of optimism over Washington-style pessimism. ... I will never accept defeat for any industry in America."
McCain had appeared to play down the state's economic problems when he said some of the hundreds of thousands of manufacturing jobs that had been lost will never come back.
Last night, he joked about the brief time, after his New Hampshire victory last Tuesday, when things suddenly turned up for his campaign. But he sounded undaunted as he addressed supporters in Charleston, S.C.
"We don't mind a fight," the Arizona senator said.
McCain was hurt by a drop in crossover votes by Democrats and independents, who helped him defeat George W. Bush in the Michigan primary eight years ago.
More than two-thirds of the Republican primary voters (68 percent) were self-identified Republicans, the exit poll found, up from 61 percent in New Hampshire last week.
Less than 10 percent of the Republican vote was cast by Democrats.
Anti-Clinton Democrats could vote "uncommitted," and Clinton's campaign sought to turn the results to her advantage last night.
"The people of Michigan and Florida [which also has a nonbinding Democratic primary] have just as much of a right to have their voices heard as anyone else," Clinton's campaign said in a statement. "Senator Clinton intends to be president for all 50 states."
Underscoring Michigan's importance for the next big contest, Huckabee and McCain watched the returns last night from South Carolina. More than two out of five of the state's likely Republican primary voters have yet to make up their minds, recent polling showed.
Michigan "can really influence the undecideds to get on a train," said Clemson University political scientist David Woodard. He said Romney's victory could produce a "resurgence" that pushes him into second place and McCain down to third in Saturday's primary.
Huckabee took a new, harder line on immigration yesterday in South Carolina. He proposed suspending immigration from countries that sponsor or harbor terrorists, claiming that the hijackers involved in the Sept. 11, 2001, terrorist attacks were in this country legally. However, the 9/11 Commission concluded that between two and 13 of the terrorists entered the United States using counterfeit documents.
Romney, who has put more than $17 million of his own money into his campaign, has resumed TV advertising in South Carolina, which he had cut back to save money. He is to return to South Carolina today.
Former Tennessee Sen. Fred Thompson is making a last stand in South Carolina and could become a contender if he finishes ahead of Romney and McCain in Saturday's primary.
Bill Lacy, Thompson's campaign manager, said in a statement last night that the former senator "remains the one true steadfast conservative in this race."
"The bottom line," Lacy said, "is that tonight is a whole new ballgame and tomorrow morning begins a brand new campaign where Republicans are looking for the clear conservative choice."
95 percent of precincts reporting
Mitt Romney 39 percent
John McCain 30 percent
Mike Huckabee 16 percent
Ron Paul 6 percent
Fred Thompson 4 percent
Rudolph Giuliani 3 percent
INSIDE -- Democratic rivals Hillary Rodham Clinton and Barack Obama blamed aides and campaign surrogates during a Nevada debate yesterday for fueling a campaign controversy over race. PG 6A