Romney is far ahead in Arizona, which also holds its primary Tuesday, but Rick Santorum has mounted an unexpectedly strong challenge in Michigan.
A day ahead of the contest, Romney veered away from his attacks on the cultural issues and went after the former Pennsylvania senator for his lack of experience with creating jobs.
"Sen. Santorum is a nice guy, but he's never had a job in the private sector," Romney said. "He's worked as a lobbyist, he's worked as an elected official. That's fine, but if the issue of the day is the economy, I think to create jobs it helps to have a guy as president who's had a job."
Meanwhile, Santorum kept up the drumbeat of promoting his conservative credentials. His campaign is actually inviting Democrats into the Republican primary to vote against Romney.
And he has been using a line of attack that's worked for him in other contests.
"I'm sure Gov. Romney getting into this race thought he would have to spend $20 and easily cruise to victory in Michigan, and here we are being badly outspent again," Santorum said.
Former House Speaker Newt Gingrich, who has faded in the race, repeated his call to repeal President Barack Obama's health care law if Republicans win congressional majorities.
He again laid out his plans to increase production of oil and drop the price of gasoline.
"You want $2.50 a gallon with Newt Gingrich or $10 a gallon with Barack Obama?" Gingrich said. "Do you want a paycheck with Newt Gingrich or food stamps with Barack Obama?"
Gingrich is planning to make a strong bid for his home state of Georgia and its neighboring southern states as he tries to keep his campaign alive.
Next week is Super Tuesday and people in 10 states are voting on a Republican candidate.