Mitt Romney warned Tuesday on Fox News that Democrats could vote in Michigan's open primary. He referred to Democratic activist and documentary filmmaker Michael Moore, who told MSNBC on Monday night that "a lot of my Democratic friends will vote for Santorum in something they are calling Operation Hilarity."

"Operation Hilarity" was a prank project announced by Democratic blog Daily Kos earlier in February that urged readers to vote for Rick Santorum in order to help drag out the Republican primary and strengthen President Obama's reelection chances.

According to exit polls, one in 10 voters in the Michigan primary were Democrats. The Los Angeles Times reported about half of those voted for Santorum.

But Romney pulled out a tight victory in what had been almost a dead heat between the two top candidates.

Wasserman Schultz maintained that Romney still has "some really serious problems" and "more and more" every day as the GOP primary continues. In a tweet, she said Romney's efforts to court conservative voters would "hurt" him in the general election.