WASHINGTON -- 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.