Now all eyes are on Super Tuesday, March 6, when primaries are scheduled in about 10 states. A major victory next week would almost seal Romney's nomination, experts say.
While final results were yet to be declared, based on the poll results so far, major media outlets here had declared Romney as the winner in both Michigan and Arizona.
"I was confident that we could come together today and take a giant step toward a brighter future," Romney said in his victory speech referring to the poll numbers last week when he was trailing behind Santorum.
Romney said his campaign is about restoring the promise of America.
"Last week, I unveiled a very bold economic plan that's going to jump-start the economy, and it's going to get Michiganders back to work, it's going to get Americans more jobs they're crying out for, and we're going to have less debt and smaller government, and I'm going to deliver on more jobs, less debt, and smaller government. We're going to hear that day in and day out, more jobs, less debt, smaller government."
Romney has so far won contests in six states: New Hampshire, Florida, Nevada, Maine, Michigan and Arizona.
"This is a decisive moment, I believe this is a time that requires real leadership in our country. Times are tough. We need leaders who will live with integrity, who have the courage to tell the truth, and have the experience to get our economy back on track. That's the kind of leader I aspire to be; that's the kind of leader I will be if I'm president of the United States," he said.