Mitt Romney accused rival Rick Santorum of dirty tricks for encouraging Democrats to defeat him in Michigan's primary on Tuesday, when voters will determine whether Romney gets a big win or a humiliating defeat in his home state.

Romney, running neck-and-neck with rival Republican presidential candidate Santorum in opinion polls, charged that his opponent was trying to kidnap the party's primary process in Michigan by appealing to Democrats to cast their votes in a contest that is open to supporters of both parties.

Romney's father was a popular governor and Romney was born and raised in Michigan. Arizona holds its Republican primary as well on Tuesday, with Romney leading in the polls.

A new poll on Tuesday underscored how the former Massachusetts governor is still viewed with suspicion by conservative Republicans.

The ABC News/Washington Post poll showed Romney had fallen to a new low among the most conservative Americans. He is viewed favourably by just 38 percent among strong conservatives, down 14 percentage points from a week earlier. Sixty percent of that group view Santorum positively.

To boost Santorum, supporters paid for an automated robocall to Democrats to encourage members of the other party to vote for the former Pennsylvania senator.

Democrats could tip the close race in Santorum's favour unless there is a big Republican turnout, in a year when the number of Republicans going to the polls 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.

Most Michigan polls close at 8 p.m. EST.

A victory for Santorum on what is essentially Romney's home turf would scar the erstwhile frontrunner a week before a defining day of the race for the nomination to take on Obama in the November 6 general election - the March 6 Super Tuesday, when voters in 10 states go to the polls.

A Santorum win in Michigan could upend the race and prompt Republican Party leaders - concerned that Santorum's unflinching religious conservatism could make him unelectable against Obama - to search for a new candidate to join the race.

I think Santorum is completely radioactive and will bring an electoral disaster to the Republicans - he could deliver Obama a landslide, said Michigan Democratic strategist Joe DiSano, who has launched one effort to help Santorum.

The Detroit News reported that the calls blast Romney for bailing out Wall Street billionaire buddies while opposing the federal auto bailout, which Obama and his union supporters have stressed as they have campaigned against Republicans.

'PLACED MY BET'

As voters in Michigan went to the polls, Obama gave a fiery speech to the United Auto Workers union, defending the bailout and trumpeting its success.

I believed in you. I placed my bet on the American worker, he said to applause and raucous 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 blue-collar Democrats, who supported Republicans like former President Ronald Reagan in earlier elections.

Rick Santorum's message - a balanced budget, cutting spending and revitalizing the manufacturing sector - resonates and inspires the conservative base and resonates with Reagan Democrats, Santorum spokesman Hogan Gidley said.

But there is nothing dirty about sending a robocall to the voters of Michigan saying we're the most conservative, we're going to fix the economy, vote for us, he said.

Romney is supported by Michigan's governor and other state party leaders, and his campaign spending here has exceeded $4 million, more than doubling Santorum's.

But Santorum has made himself competitive 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.

Santorum has the strong support of the state's evangelical Christians, an important Republican voting bloc. In national politics, Santorum is mostly strongly identified with conservative social views like opposition to gay marriage and abortion rights, for homeschooling his seven children and criticizing women who work outside the home.

Romney, seen as a moderate Republican when he was governor of Democratic-leaning Massachusetts, is viewed warily by the most conservative wing of the Republican party.

The former private equity executive, whose fortune is estimated at $250 million, has also 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.

Romney has a comfortable lead in Arizona, aided by the man who beat him in the 2008 Republican nominating race but then lost to Obama in the general election, Arizona Senator John McCain, who has endorsed and campaigned for Romney.

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)