The Republican race is tightening in Michigan ahead of the state's crucial Feb. 28 presidential primary, according to a new poll from Public Policy Polling. Rick Santorum's lead over Mitt Romney has dwindled from 15 points to just four, 37 percent to 33 percent. It's not that Santorum is falling out of favor with Michiganders — his share of the vote fell by just two percent, and 67 percent of Michigan Republicans view him favorably. So what's behind Romney's "comeback"? Here, five theories:

1. Romney's financial advantage is paying off
The lesson here, says McKay Coppins at Buzzfeed, is that we shouldn't "put stock in polls that were taken before the Romney Death Star starts pouring money into a given race." Last week's polls had everyone predicting doom for Romney in Michigan, but they were conducted before his campaign ads started airing. Now, Mitt is surging after just a few days worth of commercials, meaning "there's still plenty of time for his message to sink in and get him over the hump" for a victory next Tuesday.

2. Mitt is successfully emphasizing his Michigan roots
The Mitt surge is a "reflection of the campaigning that Romney has done in the state," says Doug Mataconis at Outside the Beltway, especially the "emphasis he's placed on his ties to" Michigan. Mitt was born and raised in Michigan. His dad, George Romney, was the state's governor and a top auto-industry executive. And interestingly, Romney's opposition to President Obama's largely successful auto bailout doesn't "seem to have hurt him among Michiganders at all," with 34 percent saying they're more likely to vote for a candidate who opposed the bailout, and only 27 percent saying opposition to the bailout is a negative. Thirty-five percent say it doesn't matter.

3. Gingrich's persistence is hurting Santorum
"Romney should send a thank you note to Las Vegas casino owner Sheldon Adelson," says Andrew Malcolm at Investor's Business Daily. Adelson has given Gingrich's super PAC almost $21 million in recent weeks, and that money is keeping Newt in the race and "keeping Romney opponents divided." If Gingrich were to drop out, 45 percent of his supporters would jump to the conservative Santorum, while only 29 percent would join Romney's camp. As long as Newt remains a factor, Romney has a good shot of taking Michigan.

4. Maybe the earlier poll was just wrong
The latest survey surely "qualifies as good news for Mr. Romney," says Nate Silver at The New York Times, "but it may overstate his momentum." Last week's poll was probably just an "outlier." It gave Santorum a bigger lead than any other survey because it exaggerated the weight of evangelical and very conservative voters, who tend to dislike Romney. PPP's latest try projects more "realistic" proportions of those frequently pro-Santorum voters. The result: The race looks more like the toss-up that it probably already was.

5. This is just a crazy race
Romney's Michigan turnaround is just the latest indication that "this is still an extremely volatile race," says Public Policy Polling. In fact, 36 percent of voters say they might change their minds between now and next week's primary. That bodes well for Romney — 69 percent of his Michigan supporters say they're strongly committed, compared to 63 percent of Santorum's. "With momentum on his side and a more reliable group of supporters, there are plenty of reasons to think Romney can continue this comeback and win."