When this year’s presidential campaign began, Rick Santorum looked like a fringe candidate, consigned permanently to the outside edge of an overcrowded debate stage. But as earlier conservative front-runners sputtered, Santorum plugged away, sticking doggedly to his unfashionable message of uncompromising social conservatism.

And then, suddenly, he got lucky. Republicans started believing the attack ads Mitt Romney and Newt Gingrich rained on each other, and both men’s popularity sank. Meanwhile, a battle over contraception coverage in health insurance plans reminded conservatives how strongly they feel about the social issues — and how much they enjoy accusing President Obama of waging a “war on religion.”

Santorum, no longer on the fringe, won two caucuses and a primary in Colorado, Minnesota and Missouri.

The resulting shift in the polls has been dramatic — the biggest five-day swing in the primary campaign to date, according to Gallup. Among Republicans nationwide, Santorum is now running neck and neck with Romney; Gingrich and Ron Paul have fallen far behind.

We’ve seen plenty of candidates rise briefly to the top in this GOP race, and so far the surges have come to nothing. But at least for the moment, we have to take Santorum seriously, especially since he’s leading the polls in the two most important primary states immediately ahead, Michigan and Ohio.

This could mean that the contest is, finally, a two-man race: Romney, the candidate of an establishment that no longer holds much sway with the base, and Santorum, the candidate of a conservative insurgency that seems able to draw voters to the polls.

So how has Santorum succeeded in getting this far? And how far is he likely to go?

Much of his rise, to be sure, has come through a process of elimination: At this point, he’s one of the few candidates for the “not Romney” role left standing.

But it hasn’t been only stubbornness and luck. He’s run a pretty good campaign, finding ways to expand his message beyond a narrow social conservative agenda. He still talks plenty about abortion and gay marriage if voters ask, but the core of Santorum’s stump speech these days is economics. Like every Republican candidate, he’s for lower taxes and a balanced budget. But he’s also proposed a set of policies to promote the creation of blue-collar manufacturing jobs, including elimination of the corporate income tax for manufacturers.

Within those themes, of course, lies an implicit critique of Romney, a Harvard-educated venture capitalist whose equally conservative economic platform is aimed at the financial markets more than the factory floor.

Not surprisingly, a CNN poll released this week found that a bit of class warfare has broken out within the GOP electorate. Republicans who describe themselves as blue collar were backing Santorum, the grandson of an Italian immigrant coal miner, by a wide margin, while those who call themselves white collar were backing Romney.

The CNN poll also pointed out another gap between Santorum and Romney voters: gender. The former senator’s uncompromising social positions — he doesn’t think anyone’s insurance policy should cover contraceptives, for example — haven’t endeared him to many women. The poll found Romney leading by 9 percentage points among Republican women, and Santorum leading by 10 points among GOP men.

Even if it’s now a two-man race, can the former senator actually win his party’s nomination? That will still be tough.

Up to now, Santorum has been spared the full force of a negative campaign, in large part because Romney and Gingrich were focused on slamming each other. But now that Santorum appears to be the biggest threat to Romney’s nomination, he’s starting to be targeted. Last week, Romney supporter and former Minnesota Gov. Tim Pawlenty denounced Santorum as a “pork-barrel spender” during his Senate terms. And the “super PAC” supporting Romney has already unveiled a television commercial that calls Santorum a “Washington insider” who voted to raise his own pay.

And Santorum doesn’t have nearly the money that Romney does. One GOP fundraiser estimated this week that Santorum has less than $2 million in cash on hand; Romney had $19 million at the end of 2011, although he spent a chunk of it last month.

Both campaigns are likely to spend heavily in Michigan, which votes on Feb. 28, because the stakes for both are high. For Santorum, a win in Michigan would prove that his surge hasn’t been a fluke and that he can beat Romney even in the state where the former Massachusetts governor was born.

Even if Santorum wins in Michigan and shows well on Super Tuesday in early March, the arithmetic will be against him. Romney leads the delegate count now, and he is virtually certain to win in several states on Super Tuesday. Santorum’s own strategist, John Brabender, has conceded that winning an outright majority of delegates may not be possible and that the insurgent’s best chance may lie in a deadlocked convention.

Republican strategists I spoke with still consider Romney the front-runner. For Santorum to keep his surge going, he’ll need to be more than just smart and lucky, the factors that brought him this far. He’ll need Romney to fail.

• McManus is a columnist for The Los Angeles Times.