In poll after poll this primary cycle, Mitt Romney has led New Hampshire, sometimes by as many as 20 to 30 points. He has courted the state, campaigning and building up a formidable organization, and has snapped up crucial endorsements from New Hampshire heavyweights such as Sen. Kelly Ayotte and former governor John Sununu. And it doesn’t hurt that he owns a home on the state’s Lake Winnipesaukee and often vacations there.

But now, armed with the New Hampshire Union Leader endorsement, Gingrich is poised to become Romney’s top rival in a state renowned for overturning conventional expectations of primary results. John McCain shot to an unexpected victory here in 2000, as did Hillary Clinton in 2008. And if the stars align, Gingrich may just be able to pull off the same stunt.

“The New Hampshire primary has long been a very rough road for a frontrunner to close here,” says Rich Killion, a New Hampshire GOP strategist who is not affiliated with any of the presidential campaigns. “And historically, for a whole multitude of reasons, New Hampshire voters decide late. If you go back and look at exit polls of the last four, five cycles, you’ll probably see anywhere between a third to 40 percent of the primary electorate will actually make up its mind on this race in the final days. As we stand right now, 65 percent of the electorate hasn’t closed on their final candidate.”

“But there’s no question Romney is not only the frontrunner, he’s a very, very strong frontrunner,” Killion adds.

Two things would likely need to happen for Gingrich to win, says University of New Hampshire political science professor Dante Scala, who describes Gingrich’s chances of winning as “plausible,” but not “probable” at this point in the race. First, Gingrich would have to win in Iowa. “If that were to happen, Gingrich would come in here with a full head of steam,” Scala remarks, noting that a Gingrich Iowa victory would set up New Hampshire to be a contest primarily between Romney and Gingrich, with Ron Paul and Jon Huntsman “being spoilers.”

“The other thing that would need to happen is that Mitt Romney would need to be brought down a peg or two in New Hampshire. Romney’s favorables have been very strong all year in all sorts of demographic groups,” Scala says.

Here again, Huntsman could play a role, nabbing enough of the moderate votes to diminish Romney’s support. Our Destiny PAC, a Super PAC supporting Huntsman that has made serious ad buys in recent weeks, could also make an impact. If the PAC decides to run a TV ad criticizing Romney, that could bruise Romney’s reputation in the state.

The other potential threat to Romney’s favorability numbers is what the Union Leader decides to do. In the last election cycle, the newspaper paired its endorsement of John McCain with a relentlessly negative series of editorials lambasting Romney. Romney has tried to woo Union Leader publisher Joe McQuaid this cycle, but McQuaid told Politico to count on the newspaper upholding its “traditional approach” this cycle.

The Union Leader’s track record on endorsements is mixed — Pat Buchanan and Steve Forbes are among those picked by the paper — but in 2008, there is no doubt the paper played a significant role in the primary.