AP

Mitt Romney is gaining momentum in Michigan against upstart Rick Santorum.

Santorum, the latest "anyone-but-Romney" candidate to surge to the head of the pack, at one point held a commanding lead in Michigan; the latest polls now have the two neck-and-neck.

A loss in Michigan, Romney's homestate, could be disastrous for the former Massachusetts governor's campaign, with some GOP insiders murmuring about the possibility of a brokered convention this summer in the event of such a loss.

Calls for a "white knight" candidate to enter the race have begun again, though the names most frequently tossed around--Jeb Bush, Mitch Daniels, Chris Christie--have remained adamant about not running. Most in the GOP admit a last minute candidacy would likely be unsuccessful.

A victory in Michigan for Romney, on the other hand, would likely give him heir apparent status once again.

Michigan and Arizona will hold their primaries Tuesday.