Mitt Romney, according to that infallible forecaster Conventional Wisdom, would be the Republican presidential nominee. Sure, he might have his hiccups, but with his background, his organization  and his money, Romney would ultimately waltz to the nomination.

Unfortunately for Romney, Conventional Wisdom forgot to clue in Rick Santorum to the plan. Suddenly, here we are, six weeks into the GOP primary, and Santorum has won just as many states as Romney - four. Making matters worse for Romney, the next state to vote - Michigan - once thought of as a Romney stronghold, since he was born there,  is now… gulp … looking as if  it's swinging toward Santorum.

A win for Santorum in Romney's birthplace, the state where his father, George Romney, was a popular three-term governor, could send shockwaves through the Republican race, put Santorum in pole position heading into Super Tuesday, and potentially spell doom for Romney's campaign, no matter what his staff in Boston says about a long, drawn-out nomination battle.

In other words, expect it to get ugly in Michigan in the next two weeks before the Feb. 28 primary. Real ugly.

As Romney proved last month in Florida, when his back seems up against the wall, he busts out a lethal arsenal. On the heels of his resounding loss to Newt Gingrich in South Carolina, Romney came into Florida desperate for a win, and he delivered. Romney delivered back-to-back strong debate performances in Jacksonville and Tampa. His Super PAC unleashed a barrage of attacks on Gingrich. And come Jan. 31, Romney ran away with the Sunshine State's  primary.

To a lesser degree in Maine, Romney came through when he needed to. After Santorum swept Minnesota, Missouri and Colorado, Romney swiftly changed the narrative by taking the Maine caucus.

Santorum should expect similar treatment in Michigan. According to new polls out of Michigan, the former Pennsylvania senator has jumped to a sizable lead in the state. Public Policy Polling has Santorum commanding a 39 percent to  advantage over Romney, with Santorum ahead in every county except Oakland, where Romney was born. An American Research Group poll shows a closer race, but with Santorum still up 33 percent to 27 percent.

"Rick Santorum has all the momentum in Michigan right now," Public Policy Polling  president Dean Debnam said.

The picture gets far bleaker for Romney if Gingrich drops out before the primary or simply fades to the point of irrelevance. According to the Public Policy Polling  survey, 54 percent of Gingrich supporters would opt for Santorum, compared with 21 percent who would go for  Romney.

On Monday the editors of the National Review went so far as to call on Gingrich to leave the race, noting, "When he led Santorum in the polls, he urged the Pennsylvanian to leave the race. On his own arguments, the proper course for him now is to endorse Santorum and exit."

So how does Romney, the home-state son and resounding favorite, suddenly find himself staring at the prospect of a loss to Santorum in Michigan?

For starters, Romney has fared dismally in the Midwest. He lost to Santorum in Iowa, Minnesota  and Missouri, all states with strong socially-conservative, working-class populations who tend to favor the former senator. Michigan is not so different, paving the way for another impressive showing from Santorum. The Public Policy poll revealed Santorum had 53 percent of the Tea Party vote there, 51 percent of the "very conservative" vote and 48 percent of evangelicals.

In addition, Romney has received considerable flak for opposing the government bailout of the struggling U.S. automakers. The auto industry dominates Michigan's economy, accounting for more than  20 percent of it, and in recent months the industry's revival has emerged as a national success story.

The former Massachusetts governor has defended his position by arguing that he believed in orderly bankruptcies for General Motors and Chrysler but not bailouts with government money. At the time, however, no private money was prepared to step in, leaving the government to choose between multi-billion-dollar bailouts  or liquidation. While Santorum opposed the bailout, too, Romney's opposition has garnered considerably more attention.

On top of all that, Romney's status as a Michigander may not help him as much as one might have thought - only 26 percent of voters consider him to be one, according to the Public Policy Polling  survey.

In the past week,  Santorum has tried to shape the race as a two-person battle between him - as the true conservative candidate - and Romney. If voters in Michigan see the race the same way, that will only strengthen Santorum's chances of emerging victorious there.

"We're just focused on making sure that folks know we're the best alternative to Barack Obama, and we have the best chance of beating him," Santorum said Sunday on CNN's "State of the Union."

That claim is bolstered by a new Gallup poll that shows Romney and Santorum in a statistical dead heat for the lead among registered Republican voters: 32 percent for the former, 30 percent for the latter, a 14 point jump for Santorum in the past week alone.

With Romney suddenly looking like a fragile front-runner and Santorum surging - he's raised more than $3 million in the past week - expect a furious fight for Michigan. According to the Public Policy Polling poll, 52 percent of voters said they could still change their minds before Feb. 28.

"It's important to note that more than 50 percent of voters say they could change their minds in the next two weeks," Debnam said. "There's a lot of room for this race to shift back toward Romney in the coming days."

Romney has already proved in Florida and Maine that he can come from behind to win - and that he knows how to use his considerable firepower. Just ask Gingrich. Plus, Romney won the state by nine points over the eventual GOP nominee, Sen. John McCain, in the 2008 primary, something that may give him hope as he heads into this month's contest.

"It was pretty darned close," Romney said on Fox News last week. "I'm not expecting a landslide."

The state's Republican Party chairman, Bobby Schostak, is a bit more optimistic about Romney's chances than the candidate himself is.

"I think that the Romney campaign will be successful in Michigan," Schostak said earlier this month. "I don't see a scenario where they're not."

Take Romney's Michigan roots, his impressive war chest and his invaluable experience. Pit it against Santorum's Midwest strength, his social conservative support  and his wave of momentum. Mix for two weeks. And expect it to boil over, because the ramifications of the Michigan primary could be felt for months to come.

Matthew Jaffe is covers the 2012 presidential campaign for ABC News and Univision.