(Reuters) - Mitt Romney faces a day of reckoning on Tuesday when Michigan votes to either grant him a big victory in the Republican presidential nomination battle or hand him a humiliating defeat.
Romney was born and raised in Michigan and his father was a popular governor, but conservatives are threatening to deliver the state to Rick Santorum, who is running neck-and-neck with Romney in the polls in the final hours before voting begins.
Most Michigan polls close at 8 p.m. EST (midnight GMT).
Arizona votes as well on Tuesday and Romney has a comfortable lead there, aided by the man who beat him in the 2008 Republican presidential campaign, Arizona Senator John McCain.
All eyes are on Michigan because a victory for Santorum on what is essentially Romney's home turf would raise questions about Romney's candidacy a week before a defining day of the 2012 campaign, March 6, the Super Tuesday when 10 states hold contests.
I am going to win in Michigan and I'm going to win across the country, Romney said on Monday.
His aides, contemplating the possibility of a defeat, believe Romney could survive a loss in Michigan should it occur because of this year's elongated nomination process.
The bottom line is you want to win it but it is not as devastating as you guys want to make it out to be if we don't, said a senior Romney strategist.
Santorum, a former Pennsylvania senator, has made himself competitive in Michigan by pressing his conservative views on social issues and by spreading a blue-collar message about the need to rebuild the manufacturing base in the hard-hit Midwestern state.
We've been traveling all over the state, and I'm really excited about the response. I think we're going to surprise a few people tomorrow night, Santorum said on Monday.
A Santorum win could upend the race and prompt the Republican establishment - concerned that Santorum's strong religious conservatism could make him unelectable - to search for a new candidate to join the race.
An unpredictable factor in Michigan was the ability of Democrats to vote in the Republican primary and try to thwart Romney by voting for Santorum, who many see as having little chance of defeating Democratic President Barack Obama in the November 6 election should he become the Republican nominee.
The Santorum campaign tried to encourage the crossover vote with a robocall urging Democrats to send a message to Romney because of his opposition to 2009 auto bailouts that kept thousands of Michigan workers employed.
The effort was quickly condemned by the Romney campaign as a sign that Santorum is now willing to wear the other team's jersey if he thinks it will get him more votes.
Romney has come back from a deficit in the Michigan polls to creep ahead in some surveys, and his aides believed his campaign has the organizational strength for a good turnout.
Other Republican candidates, Newt Gingrich and Ron Paul, are running far behind the two leaders and have not competed heavily, making the state a Romney-or-Santorum contest.
Romney has been hammering home his view that his experience as a private equity executive and former Massachusetts governor makes him the best candidate to defeat Obama and lead the U.S. economy back to strong job growth.
He has also been sharply critical of Santorum.
I've spent 25 years in business, Romney said. I understand why jobs go, why they come. I understand what happens to corporate profit, where it goes if the government takes it. This is what I've done for all my life. Senator Santorum is a nice guy, but he's never had a job in the private sector.
This kind of message is resonating among many Michigan Republicans.
He could be more charismatic but a steady, good businessman is what we need, said John Bas of Berkeley. Is he a rock star? No. But rock stars probably don't make good presidents.
Romney may not be a rock star, but he had one campaign for him on Monday. Kid Rock and his band joined Romney at a theatre in Royal Oak to play a song that is the signature anthem of Romney's campaign events, Born Free.
Romney and his wife Ann watched the brief concert from the front row and clapped to the beat.
Romney persuaded Kid Rock to perform at the rally during an hour-long meeting last Thursday at the rocker's Michigan home. Kid Rock got Romney's commitment that if elected he would help the city of Detroit, the state of Michigan and U.S. troops, a Romney campaign spokesman said.
(Additional reporting by Sam Youngman; editing by Christopher Wilson)