SOUTHFIELD/GRAND RAPIDS, Michigan (Reuters) - Mitt Romney condemned rival Rick Santorum for urging Democrats to vote against him and "kidnap" the Michigan Republican presidential primary on Tuesday, as voters determined whether Romney would get a big win or a humiliating defeat.
Santorum, running neck-and-neck with Romney in opinion polls, is trying to snatch an upset win in his rival's home state, where the Republican primary is open to both parties.
His campaign appealed for Michigan Democrats' votes with an automated "robocall" bashing Romney, a wealthy former private equity executive, for failing to support President Barack Obama's federal government bailout of the U.S. auto industry.
"Romney supported the bailouts for his Wall Street billionaire buddies but opposed the auto bailouts. That was a slap in the face of every Michigan worker and we're not going to let Romney get away with it," the call said.
The stakes are particularly high for Romney in Michigan, the home of the U.S. auto industry, where he was born and raised and where his father was a popular governor in the 1960s.
A victory for Santorum would scar the erstwhile front-runner a week before a defining day for the Republican nomination to take on Obama in the November 6 general election: the March 6 "Super Tuesday" contests in 10 states.
Aides said Romney has the funds and organization to win his party's nomination even if he loses Michigan.
But a Santorum win in the Rust Belt state, on the heels of his victories in Colorado, Minnesota and Missouri earlier this month, could upend the race. Republican Party leaders, concerned that Santorum's unflinching religious conservatism could make him unelectable against Obama, may feel pressured to search for a new candidate to join the race.
Arizona also holds its Republican primary on Tuesday, with Romney well ahead in the polls.
Santorum had trailed far behind Romney and several other Republicans vying for the nomination until recent weeks.
But other contenders have faltered, and the former Pennsylvania senator has leaped to the top of the pack, despite trailing in fundraising and a skeletal campaign staff.
When asked if he would win on Tuesday during a stop at his Grand Rapids headquarters, he shrugged and quipped: "I'm not a pollster. We don't even have a pollster."
In Michigan, Santorum made himself competitive against Romney by painting Romney as a moderate and stressing his own conservative views on social issues, blue-collar roots as the grandson of a coal miner and vision for rebuilding manufacturing in the hard-hit Midwest.
An ABC News/Washington Post poll on Tuesday underscored some conservatives' lingering worries about Romney, who was seen as a moderate when he was governor of Democratic-leaning Massachusetts six years ago.
The survey found Romney at a new low among the most conservative Americans. He is viewed favorably by just 38 percent of that group, the poll showed, down 14 points from a week earlier, while 60 percent view Santorum positively.
Phoenix Telephone company worker Thomas Skriba voted for Santorum in Arizona's primary because of his conservative views.
"I'm not really happy with any of the candidates totally, but he more closely stands for the values that I stand for in my Christian faith," he said. "I did not side with Romney because of his voting record. It seemed a little bit liberal to me."
Democrats could tip the close race in Santorum's favor unless there is a big Republican turnout, and the number of Republicans participating in primaries and caucuses has been declining steadily compared to the 2008 campaign.
Romney said the robocall helps make the race unpredictable.
"There's a real effort to kidnap our primary process, and if we want Republicans to nominate the Republican who takes on Barack Obama, I need Republicans to get out and vote and say no to the dirty tricks of a desperate campaign," he told a news conference at his headquarters in Livonia.
Michigan polls close at 8 p.m. EST (0100 Wednesday GMT).
Obama marked Michigan's primary day with a fiery speech to the United Auto Workers union, defending the auto bailout and trumpeting its success. Romney has repeatedly voiced his opposition to organized labor as he has campaigned.
"I believed in you. I placed my bet on the American worker," Obama said to applause and cheers at the union's convention. "Three years later, the American auto industry is back.
Santorum's campaign said the robocalls were an effort to reach out to conservative Democrats, who supported Republicans like former President Ronald Reagan in earlier elections.
"Hopefully we'll appeal to a lot of blue-collar Democrats in this state. I welcome their vote, and I welcome anybody's vote who shares my values," Santorum told reporters.
Romney is backed by party leaders in Michigan, and his campaign spending here has been more than double Santorum's.
Santorum has the strong support of the state's evangelical Christians, an important Republican voting bloc. In national politics, Santorum is most strongly identified with opposition to gay marriage and abortion rights, homeschooling his seven children and criticizing women who work outside the home.
The former private equity executive Romney, whose fortune is estimated at $250 million, has struggled to convince voters that he can relate to the struggles of average Americans and made gaffes reminding Americans he is among the super-rich.
Other Republican candidates - Newt Gingrich, a former speaker of the U.S. House of Representatives, and Ron Paul, a Texas congressman - are running far behind the two leaders and have not competed heavily in Michigan, making the state a Romney-or-Santorum contest.
Additional reporting by Eric Johnson in Michigan, Tim Gaynor in Arizona, and Eric Beech, Susan Heavey and Patricia Zengerle in Washington; Writing by Patricia Zengerle; Editing by Vicki Allen and Doina Chiacu