Similarly, victory in Michigan could revive Mitt Romney, the wealthy former Massachusetts governor who was stunned by twin defeats, in neighboring New Hampshire to McCain and in Iowa to the underfunded Mike Huckabee, the former Arkansas governor.
Romney, son of a Michigan governor, poured nearly everything he had into Michigan, moving resources from two critical states that vote next, South Carolina and Florida. A recovery by Romney would keep the Republican field in disarray without any potential favorite.
Early exit polls indicated that Democrats would be less of a factor in the GOP primary than eight years ago, when they helped McCain win Michigan over President Bush. Michigan has an open primary and no registration by party.
Preliminary surveys taken for The Associated Press and the television networks showed:
-Fewer than one in 10 voting in Tuesday's GOP primary were Democrats, compared with 17 percent in 2000.
-A quarter of voters in the GOP primary were independents, down about 10 points from 2000.
-The proportion of Republicans voting in the GOP primary rose from just under half in 2000 to two-thirds on Tuesday.
All three men jockeyed to promise extra support for Michigan, where the auto industry's downturn has pushed job losses well above the national rate.
They offered scant specifics and ruled out a government bailout, but they made general promises of lower taxes and less regulation and more job training and greater investment in research and technology. And Romney and Huckabee disputed McCain's contention that some lost jobs will never return to Michigan.
Indeed, far more Republican voters in Michigan rated the economy as the most important issue facing the country than in the New Hampshire or Iowa contests, exit polls showed. Given four choices, half of Michigan Republican primary voters picked the economy, compared to just 26 percent in Iowa and 31 percent in New Hampshire.
On other issues, one in five GOP voters picked Iraq, one in seven chose immigration and one in 10 called terrorism the country's most important issue.
With the history Romney and McCain have in Michigan - McCain won the primary here in 2000 - there was less oxygen left for Huckabee. But the former Baptist minister hoped that his strongest supporters to date, born-again Christian voters, would turn out along Michigan's conservative western edge to help him finish a strong third or better.
Huckabee's chances are better in the next state to vote, South Carolina, where conservative Christians dominate the GOP. He is vying for their support with actor-politician Fred Thompson. Despite his strong debate performance last week, Thompson has kept a lethargic campaign schedule in the final days of the contest, especially compared to Huckabee's rallies and two sermons at a Spartanburg megachurch on Sunday.
The winner, or winners, of Michigan and South Carolina should have an edge going into Florida in two weeks, where polls show the race narrowing to a four-way tie among former New York Mayor Rudy Giuliani, McCain, Huckabee and Romney.
Giuliani has been camped out in delegate-rich Florida since he abandoned efforts to compete in earlier states; he spent millions of dollars in ads and mailings in New Hampshire and now Florida.
"A Romney loss in Michigan pretty well cripples Romney," said GOP consultant Rich Bond. "A Thompson fourth-place showing in South Carolina pretty well finishes off Fred Thompson. And then what is most likely, if Romney doesn't win Michigan, is three players all with varying degrees of viability - McCain, Giuliani and Huckabee."
