Mitt Romney is challenging Rick Santorum's conservative principles as he campaigns across his native state of Michigan today.

Romney points to a series of votes the former Pennsylvania senator says he now regrets.

Santorum calls Romney's claims "laughable" and says conservatives shouldn't trust his rival for the GOP presidential nomination. He calls Romney an "Occupy Wall Street adherent" because his tax plan would limit deductions and exemptions for "the top 1 percent" of income earners.

Michigan and Arizona hold primaries on Tuesday.

Santorum calls the race in Michigan, the state where Romney was born and raised, "close" and "winnable."

In the meantime, Saturday, Democratic governors are bullish on President Barack Obama's re-election prospects. They point to the improving economy and a Republican primary campaign that's exposed deep divisions within the party.

Republicans governors, meanwhile, insist Obama is vulnerable, but they say they're concerned the prolonged campaign has alienated independent voters and may have badly damaged the eventual nominee.

Democratic enthusiasm and Republican apprehension are on display at the winter meeting of the National Governor's Association going on in Washington. That's the annual four-day conference where governors discuss policy and trade ideas, and where politics are always just below the surface.

In interviews, many Democratic governors seemed almost giddy about Obama's chances of winning a second term. Virtually no Republican governors were willing to predict a victory for their party's nominee in November.

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