Polls show that former Massachusetts Gov. Mitt Romney and former Pennsylvania Sen. Rick Santorum are running neck and neck in Michigan.
The stakes are high for both candidates.
A home-state loss for Romney would severely damage his campaign, and open up the race just one week before Super Tuesday.
As a result, both candidates stumped hard in the state as voters hit the polls.
"I think he's an economic lightweight. I don't think he understands the process of job creating," Romney said about his now closest opponent.
Meanwhile, Santorum told crowds that their vote for him could shake up the race.
"Do something big. Shock the establishment," he urged.
Click play to watch Heather Sells' report, followed by comments from Charles Dunn, Regent University distinguished professor of government.
But some of Santorum's eleventh hour campaign tactics are proving to be controversial. One robo-call by his campaign tells Michigan Democrats -- who can declare that they are Republican for Tuesday's primary -- to vote against Romney.
"Romney supported the bailouts for his Wall Street billionaire buddies, but opposed the auto bailouts," a voice on the call says. "That was a slap in the face to every Michigan worker and we're not going to let Romney get away with it."
But the message leaves out the fact that Santorum himself also opposed the auto bailout.
Romney's campaign called the tactic outrageous.
"It's confusing to people. It's a new low in this campaign," Romney said.
Romney and Santorum have spent little time in Arizona, which also held its primary Tuesday. Romney is expected to handily win the state, in part because of its large Mormon population.
Neither former House speaker Newt Gingrich or Rep. Ron Paul are actively competing in Arizona or Michigan. Instead, they're shoring up support for the 10 states that vote next week on Super Tuesday.
The key voting day will have 419 delegates up for grabs.
So far, Romney has 123 delegates. Santorum is in second with 72, followed by Gingrich at 32 and Paul at 19.
To win the GOP nomination, 1,144 delegates are needed.