Romney went after Santorum's image as a true conservative on Thursday.
"He voted to fund Planned Parenthood even though that was against his principles," Romney said.
The former Senator looked like a classic Washington politician.
In last night's debate he listed measures he opposed but voted for, like No Child Left Behind.
"It was against the principles I believed in, but you know, when you're part of the team sometimes you take one for the team for the leader and I made a mistake," Santorum admitted.
"I wonder, which team he was taking it for?" Romney fired back.
Santorum, meanwhile, charged Romney changed his stand on abortion in a new ad to run in Michigan.
With the focus on Michigan, President Obama put up an ad there saying he saved U.S. automakers.
Meanwhile, the president mocked the Republican plan to lower gasoline prices.
"Step one is drill, step two is drill, and step three is keep drilling," Mr. Obama said. "
Well the American people aren't stupid. The know that's not a plan - especially since we're already drilling."
Whoever takes Michigan Tuesday gets a big boost into Super Tuesday March 6th.
Roughly half the delegates needed for nomination are at stake in the next two weeks.