Of all the polling that takes place over the next three weeks, Michigan might get the highest profile.  Mitt Romney has strong ties to Michigan; his father was a popular governor in the state, and most people assumed Romney would not have to expend much energy there to win a Republican primary.  The first hint of trouble came two weeks ago in a Rasmussen poll that showed Romney only 15 points up over Newt Gingrich but only at 38%, roughly what he got in 2008 against two strong challengers.  I wrote at the time that Michigan could provide an opening for an unpleasant surprise for Team Romney, and today’s ARG poll of 600 likely Republican primary voters has delivered it:

Rick Santorum leads the Michigan Republican presidential primary with 33%. Santorum is followed by Mitt Romney with 27%, Newt Gingrich with 21%, and Ron Paul with 12%.

Of course, this could be an outlier, as none of the previous polling in Michigan has had Santorum out of the teens. Now, though, Santorum takes 42% of the Republicans surveyed in the poll (72% of the sample), with Gingrich coming in a distant second at 24%.  Romney wins nearly a majority of independents at 48%, with the other three candidates in a virtual tie in the teens.  Santorum now leads among Tea Party adherents 37/29 over Gingrich, and comes in a close second to Romney among non-TP adherents 35/30, with Gingrich at 14%.  Romney tops Santorum among women only by six points, 39/33, while Santorum beats Gingrich among men 33/28, with Romney at 17%.

Team Romney had better hope that this is an outlier.  A Michigan loss would seriously damage Romney’s electability argument, and would give Santorum a great deal of momentum heading into Super Tuesday.  Is it an outlier, though? PPP tweeted yesterday that their multiple-day survey in the state so far showed Santorum up by as much as 10-15 points over Romney, and has Newt Gingrich losing to Ron Paul.  Those results just got published, and Santorum leads 39/24:

Rick Santorum’s taken a large lead in Michigan’s upcoming Republican primary. He’s at 39% to 24% for Mitt Romney, 12% for Ron Paul, and 11% for Newt Gingrich.

Santorum’s rise is attributable to two major factors: his own personal popularity (a stellar 67/23 favorability) and GOP voters increasingly souring on Gingrich.  Santorum’s becoming something closer and closer to a consensus conservative candidate as Gingrich bleeds support.

The internals look different than ARG’s, however:

Santorum’s winning an outright majority of the Tea Party vote with 53% to 22% for Romney and 10% for Gingrich. He comes close to one with Evangelicals as well at 48% to 20% for Romney and 12% for Gingrich. And he cracks the 50% line with voters identifying as ‘very conservative’ at 51% to 20% for Romney and 10% for Gingrich.

Santorum’s benefiting from the open nature of Michigan’s primary as well. He’s only up by 12 points with actual Republican voters, but he has a 40-21 advantage with the Democrats and independents planning to vote that pushes his overall lead up to 15 points. Santorum is winning by a healthy margin in every region of the state except for Oakland County, where Romney has a 40-26 advantage, and the area around Lansing where Paul actually has an advantage at 30% to 27% for both Romney and Santorum.

As noted, Paul actually finished ahead of Gingrich in the PPP poll, but only by a single point, which is a virtual tie.  How did Gingrich fare so poorly in Michigan?  His favorability rating dropped into negative territory at -9, with only 38% seeing him favorably.  He does better than Paul’s -19, but Romney has a +10 and Santorum a huge +44 at 67/23.

We’ll see if other polling confirms ARG and PPP and corroborates the assessment of PPP’s president, who writes that “Rick Santorum has all the momentum in Michigan right now.”  If so, Romney will have to spend a lot of time and money in the state to get it back in his column, or his electability argument will evaporate just in time for the biggest date on the primary calendar.