In a Republican presidential race in which no candidate has ever been able to hold onto a lead for more than a couple of weeks, it has been difficult to tell whether Rick Santorum’s recent surge would last until next week’s crucial Michigan primary. Santorum’s star has been rising ever since he swept the February 7 trifecta in Colorado, Minnesota and Missouri. But after a few days in which his hard line stands on social issues started to become the focus of mainstream media attention, what happens in the next week will tell us a lot about whether the Pennsylvanian has what it takes to become his party’s presidential nominee. The first indication that the Santorum tide may be ebbing a bit came yesterday with a Public Policy Polling survey that shows his lead in Michigan might be slipping.

PPP’s previous Michigan poll was an outlier in that it gave Santorum a 15-point lead in Romney’s birthplace, far more than others taken in the state (though all had Santorum ahead in the race). So Romney’s camp may take heart from the Democratic-leaning firm’s latest effort that shows him down by only a 37 to 33 percentage-point margin. Though PPP’s breakdown of the numbers doesn’t seem to show much leakage for Santorum because of the abuse he’s been taking about his views on religion and sex, Romney’s intensive campaigning in Michigan seems to have improved his numbers there. The question for Santorum is whether he can maintain his momentum now that he, rather than his opponent, is in the glare of the spotlight.

The good news for Santorum is his net favorability numbers in Michigan have remained positive despite the increasingly negative attention his views on contraception and other social issues are starting to get. He’s still viewed positively by 67 percent of Republicans and negatively by only 23 percent. The difference between this week’s poll and last week’s is that for some reason Romney’s favorability has gone up by 10 points in that time. That’s a vital statistic for Romney. Up until now, it has been assumed the only way for him to do well was by trashing the reputations of other candidates as he did with Newt Gingrich in Iowa and Florida.

Romney has a huge lead over Santorum in money, a better organization and a truly national campaign that can be competitive across the country. But up until the recent spate of stories that have sought to portray Santorum as a rabid social conservative, Romney had seemed to be about to go into a free fall. Indeed, the latest Gallup tracking poll shows him trailing the former senator by a 36-28 point margin nationally.

But Romney has two key factors in his favor in the upcoming week.

One is the sheer volatility of the GOP race. Republicans have changed their minds more in the last nine months than anyone could have thought possible. Frontrunners have come and one with a shocking rapidity that should have taught political pundits to be wary of assuming that just because one candidate is up now that he will still be in the driver’s seat a week later. Santorum’s surge has been the result of dazzling timing that capitalized on his personal life (his daughter’s illness that generated sympathy for the candidate and shown a light on his image as a family man), the way Romney and Newt Gingrich eviscerated each other in dueling negative ad campaigns and President Obama’s attack on the Catholic Church that concentrated GOP minds on social issues. But a week can be a lifetime in politics. If the focus on abortion, gay rights and contraception helped Santorum in one sense, it is his Achilles’ heel in another as the media’s demonization of him can serve to remind Republicans that he will be brutalized by the left on these issues in a general election.

The other factor that gives Romney hope he can turn his current difficulties around is Newt Gingrich’s persistence. Gingrich may be motivated in large measure by his hatred of Romney as much as by his own ambition. But by hanging around even after it has become clear Gingrich is an afterthought in the race (he has fallen far behind and PPP has him in dead last trailing Ron Paul for third place by 5 percentage points for third place), the former speaker may prevent Santorum from sweeping the upcoming primaries. Romney’s success so far has been based on a divided conservative field, and so long as Gingrich stays in even at a diminished level, he makes it much more difficult for Santorum to win.

If the other Michigan polls in the next few days show a similar swing back to Romney, it may be a sign Santorum has peaked a bit too soon. The former Massachusetts governor may well weather the most difficult challenge to his candidacy yet.