The central narrative of the Republican nomination contest is easy to summarize: Any candidate who is perceived as the main opponent to Mitt Romney immediately ties or leads Mitt Romney.

Rick Santorum’s surge tracks with recent precedent. His support is about the same as Rick Perry’s at his peak. A little higher than Herman Cain’s crest. A little lower than Newt Gingrich’s pinnacle.

But Santorum is not only Romney’s latest challenger, he is the most serious. Perry did not possess presidential-level skills. Cain lacked any apparent qualification for high office. Gingrich managed to systematically confirm every doubt about his style and stability.

Santorum, in contrast, has shown the ability to learn. While his initial debate performances were peevish and unappealing, he has grown more confident and likable over time. He has effectively prosecuted Romney’s public record while avoiding anger or overreach. (He pointedly refused, for example, to attack Romney’s business achievements and personal wealth.)

The former Pennsylvania senator possesses strengths that neatly fit some of Romney’s weaknesses. Santorum combines a deeply held social conservatism with an authentic blue-collar appeal. Romney has trouble competing in either category. While Santorum is very conservative, he avoids being a conservative caricature. He was one of the Senate’s main advocates of global health programs and a champion of faith-based anti-poverty efforts.

And Santorum has an additional advantage over Gingrich as the anti-Romney. The GOP establishment — party types and elected Republicans — viewed the prospect of Gingrich’s nomination with undisguised horror. Having worked with him, they did everything they could to defeat him — a revealing commentary. Santorum is hardly the party favorite, but establishment objections are many degrees less heated.

Santorum, like any suddenly emerging challenger, is a blank canvas on which the Romney campaign will write. It has already made large ad purchases, which are not likely to feature positive Romney bio spots. But when it comes to negative attacks, the Romney campaign does not yet have Santorum’s number. Santorum did vote for earmarks and a congressional pay raise. But these crimes against conservatism pale in comparison to Romney’s own. Santorum supported debt-ceiling increases. But this distasteful legislative responsibility has also been performed by most of Romney’s congressional allies. Santorum is no libertarian, but neither is Romney. On the size and role of government, Romney has a serious log-in-his-own-eye problem.

And Romney is unable to directly exploit Santorum’s main electoral weakness — his occasional, off-putting relish for the culture wars. Santorum has gone out of his way to question the role of women in the workplace and in the military, and emphasize his opposition to contraception. “One of the things I will talk about,” he said in October, “that no president has talked about before, is, I think, the dangers of contraception in this country.” There is a reason no president has ever done this: because some of the staunchest moral conservatives in America — people who are strongly pro-life and committed to the protection of religious liberty — consider contraception to be morally permissible. A presidential candidate should aspire to lead the country, not the splinter of a movement.

Santorum’s conservatism has some jagged edges, which has elevated Romney’s polling numbers among women. But it is difficult for Romney himself to press this case without sounding like a Massachusetts social liberal. He can only employ proxies and raise the general issue of electability. Barack Obama would be under no such constraints.

Romney has entered a high-stakes expectations game. A narrow loss in Michigan, combined with a convincing win in Arizona, would probably be survivable. A number of good showings on Super Tuesday, along with clear victories in delegate-rich primaries down the road such as New York and California, would likely be enough to make him the nominee. But a humiliating loss in Michigan would shatter Romney’s fragile front-runner status. At that point, an advantage in money would mean little. A purely financial argument for inevitability means that a candidate is not inevitable.

Which highlights Romney’s deeper problem. His campaign is very good at tactics. It has taken each challenger, found his weakness and pounded it home. But Romney’s candidacy remains short on aspiration. His public appeal, at this point, is a combination of emphasizing his business experience, criticizing Obama’s record and reassuring conservatives. This is a campaign — but not a cause.