Santorum's surge means new strategy for Romney

Romney has also planned a more aggressive campaign schedule in Michigan in the coming weeks. He will hold events in Kalamazoo and Grand Rapids on Wednesday and stay in the Midwest through the end of the week. He's likely to spend some time campaigning in Ohio, which holds its primary on March 6, Super Tuesday, and is the first Rust Belt state to hold a nominating contest.

Romney's big advantage is money. He and his allies, the super PAC Restore Our Future, have spent a combined $25 million on TV ads to date, helping to drive wins in New Hampshire, Florida and Nevada. That dwarfs the $7.1 million Gingrich and his allies have spent on airtime and the $2.5 million Santorum backers have shelled out. 

Still, Romney is facing a crush of primaries and caucuses on March 6, when his financial edge will be tested. But he always could add to that himself. He hasn't said if he'll contribute any of his considerable personal fortune to the campaign. In 2008, he spent $45 million.