Rick Santorum and Mitt Romney are sharply challenging one another’s conservative principles as they charge into the airtight primary contest in Romney’s home state — a critical election that has compelled Santorum to drop a bit of his nice-guy demeanor.

Santorum, who campaigned across Michigan Saturday, has used campaign and media appearances to lambaste Romney as a big-spending, big-government, “pro-choice gay-rights supporting” candidate — at least when he was governor of Massachusetts.