Mitt Romney’s victories in the Maine Republican caucus and the CPAC straw poll may have, as I wrote yesterday, stopped the bleeding after a week in which Rick Santorum’s sweep of three states halted the frontrunner’s momentum. But a pair of polls from the Democratic-leaning Public Policy Polling firm as well as one from the American Research Group ought to be scaring the pants off of Romney’s camp. As I wrote yesterday, PPP’s national poll of Republicans shows, in distinction to other surveys, Santorum taking a huge lead over Romney in the wake of his wins last Tuesday. But far more interesting are new polls of Michigan Republicans that also show Santorum leading Romney in his home state. A PPP Michigan poll shows Santorum leading there by a 39-24 percent margin. The ARG poll also has him ahead, though by a smaller 33-27 percent differential.

Bigger leads than that have evaporated in less time than the two weeks Romney has to make up this deficit, so there is no reason for him to panic. But if any in his campaign were inclined to dismiss Santorum’s surge as a mere bump in the road, this poll is proof they are dead wrong. Despite Romney’s huge advantage in money and endorsements as well as delegates, a loss in Michigan would be fatal to his presidential hopes. If he is beaten there, the air would go out of his inevitability balloon and Santorum would, despite his threadbare national campaign, assume the unlikely role of frontrunner.

The danger for Romney in Michigan is a defeat there would be considered a personal repudiation of his candidacy in a state that has long been assumed to be in his pocket. Were that to happen, his campaign would fall apart like a house of cards.

There are four obvious reasons for Santorum’s surge in Michigan.

First is his likeability factor. As I’ve written previously, the Pennsylvanian’s decision to avoid getting into the middle of the Gingrich-Romney mud-wrestling match helped position him as the good guy in the race. So did the sympathy that arose for Santorum after his daughter’s illness, that highlighted his status as a family man.

Second is his appeal to working class voters. Many conservatives may have scoffed when Santorum spoke of the need for Republicans to go back to appealing to working class Reagan Democrats via economic populism and made an appeal to bring back labor-intensive industries to the Rust Belt. But whatever the virtues of this approach, it is designed to work well in a state like Michigan where such voters abound.

Third, is the new importance being given to social issues in the campaign as a result of President Obama’s attack on the Catholic Church on the issue of forcing religious institutions to pay for insurance coverage of contraception. Santorum’s image as a scourge of the gay community may be a liability in a general election, but it can do him no harm in a Republican primary where social conservatives and evangelicals predominate because Romney cannot compete with Santorum in pleasing such voters. Indeed, his flip-flops on such issues are a distinct liability. An election in which GOP voters are thinking about abortion, gay marriage and even contraception insurance coverage is one in which Romney is bound to lose.

Fourth, is the collapse of Newt Gingrich. The former speaker is roaming the countryside seeking new donors to keep his candidacy going now that Sheldon and Miriam Adelson have stopped writing him checks. He’s hoping he can stay in until Super Tuesday, when victories in southern states might give him new life. But he appears to be bottoming out in the polls. In some cases, as in PPP’s Michigan poll, he’s even fallen behind Ron Paul. If this trend continues, it will leave Santorum as the only viable conservative left in the race. That’s a formula Romney, who rose to the top because of the division of the conservative vote, must fear.

Of course, as we have witnessed several times during the GOP race, Santorum’s momentum can vanish as quickly as it gathered steam. Attacks on his record in the Senate may decrease his popularity. Gingrich could mount a comeback (as unlikely as that may seem, he’s risen from the dead twice already). And Romney could shake off his lethargy, quit making gaffes and blatantly insincere statements that do more to alienate conservatives than engage them such as his claim to have been a “severely conservative” governor of Massachusetts.

Perhaps an avalanche of campaign spending by Romney and the realization that nominating a culture warrior will doom GOP efforts to win independents in the fall will reverse Santorum’s momentum. A lot can happen in two weeks and probably will. But if Romney doesn’t get his act together in Michigan, he’s in bigger trouble than he realizes.