According to the latest polls, Rick Santorum is now the Republican frontrunner. National polls have him ahead of Mitt Romney, and he’s far ahead of the former Massachusetts governor in the all-important Michigan primary. This surge, however, puts Santorum in the most dangerous spot in the GOP race, directly in front of Romney. It’s a spot that has been occupied a number of times before and all who have been there are now struggling—like Newt Gingrich—or out of the race—like Rick Perry.

The reason all of Romney’s previous frontrunner rivals have faltered is simple: the overwhelming force of the negative ads that Romney, and his allied super-PAC, Restore Our Future, have run against them. In fact, Romney has run so many negative television commercials that it may already be diminishing his favorable standing among voters. But while these ads may be hurting Romney, they’ve hurt his opponents far more. Santorum has not had any “oops moments” or “grandiose ideas,” but there’s still a lot of fodder for Romney and his allies to use to go after him.

The first line of attack is likely to be Santorum’s record in Congress and time in Washington. He served four years in the House and 12 in the Senate and voted thousands of times in the process. He’s considered particularly vulnerable for his aggressive pursuit of earmarks, comparatively pro-labor stance on such issues as “right to work,” and raising the minimum wage—along with his very tenure in Washington.

This seems to be the initial approach taken by the Romney campaign. On a conference call with press yesterday, former Missouri senator and Romney supporter Jim Talent attacked Santorum as “a member of the liberal wing of the Republican Party” and someone whose life experience was “shaped primarily about politics.” Although Talent conceded that Santorum had strong credentials as a social conservative, he laid into him on economic issues, attacking him for voting for bills such as No Child Left Behind and Medicare Part D, which were big expansions of federal power, and for supporting pro-labor legislation that “intru[ded] on employer rights.” He followed this by citing a Cato Institute attack on Santorum for “never meeting an earmark he didn’t like.”

The advantage of this attack is that it offers the necessary contrast for Romney’s campaign, according to leading Democratic strategist Tad Devine. In Devine’s eyes, Romney’s strengths are his economic background and the fact that as Missouri state auditor and Romney supporter Tom Schweich said yesterday, “Romney has never lived a day in Washington, D.C.” By hitting Santorum on these issues, Devine says, not only will Romney make attacks that resonate among GOP primary voters, he also will be able to launch a strong counterpunch to any reply from Santorum.

GOP consultant Rick Wilson agrees that the most effective attack for Romney may be to go after Santorum’s time inside the Beltway. He said he would recommend that the Romney campaign “keep up the drumbeat on Rick Santorum and the big-government culture of Washington.” The credential of having a lot of Washington, D.C., experience is not “a calling card” among voters any more.

Romney also may go after Santorum for being “unelectable.” After all, Santorum lost his last Senate race in 2006 by a near-landslide margin of almost 20 points, and has only been able to win in caucus states or uncontested beauty-pageant primaries so far. In contrast, Romney boasts of his well-oiled political machine, which is well-funded and has ensured that—unlike his competitors (save for Ron Paul)—his name is on the ballot in every GOP primary. In fact, one of the keys to Romney’s success so far has been his support among that segment of GOP primary voters who are most concerned about beating President Obama in November.

Santorum’s appeal has been centered around social conservatives, and he has had to squeak by with a ragtag political operation that has been so cash-poor that he was still soliciting Iowa Republicans for money on the eve of the caucuses. Romney certainly will emphasize the strength of his own campaign. He will ask voters: who is more likely to defeat Barack Obama—a Republican who could win even in Massachusetts, or a candidate who was blown out the last time he ran for office.

Focusing on electability would be the counsel Democratic strategist Hank Sheinkopf gives the Romney campaign. He sees the Romney campaign stuck in perilous waters right now. To Sheinkopf, “the only line of attack is that only Romney can win, and the problem is that it hasn’t worked so far” On the issues, he thinks Santorum is a much better fit for “lunch-bucket Republicans” as opposed to Romney, who is the candidate of “the poobahs.” Bob Ehrlich, the former Maryland governor who is chairman of the Romney campaign in that state, agrees that “electability” is Romney’s advantage. While Santorum “is a serious competitor,” Romney is the only one in the field with “the views and the résumé and the organization” necessary to win, he says. To Ehrlich, voters aren’t concerned about time in Washington as much as “competence.” And, in his eyes, that’s what makes Romney stand out.

There is one more broad area of vulnerability for Santorum, and this is one that there will not be any negative ads about—nor lots of time spent by the Romney campaign. They won’t need to devote much time; that’s what late-night comedians and the Internet are for. While Rick Santorum’s social-conservative credentials are unquestioned, they have led to him making controversial comments, including criticizing “radical feminists” for encouraging women to enter the workforce, and comparing homosexuality to “man-on-dog” sex. Fortunately for him, these may cost him among independents and swing voters in a general election but are unlikely to wound him in a Republican primary.

For now, it seems clear that the Romney campaign is taking the first path, of attacking Santorum for his Washington experience and economics apostasies. But even this holds risk, since, as Sheinkopf points out, voters have a difficult time holding two ideas in their head at once, and it may be difficult to convince them that the avowed social conservative is simultaneously a liberal. And after all, Republican primary voters are still having a hard time trying to grasp the notion of Mitt Romney as “severely conservative.”