Less than 24 hours before the Iowa caucuses got under way, Mitt Romney, paragon of discipline, stood among empty oil drums and rusting tools in a Marion asphalt plant and let himself get a little carried away. “We’re going to win this thing!” he declared to a cheering throng. His staff scrambled to tamp down this impromptu bit of expectations-setting. But his elation was understandable. The growing crowds validated his last-minute decision to compete all out in a state that four years earlier had spurned him, and he sensed a big win coming. And while his eight-vote squeaker didn’t provide the decisive margin he would have preferred, Romney still looks to have a clear path to the Republican nomination.

His win is impressive in two regards: In a state famous for the influence of its social conservatives, he finished ahead of Rick Santorum, the candidate best positioned to benefit from their support; and after two years during which the small-government, antitax activists of the Tea Party shaped the contours of Republican politics—and ended the careers of several distinguished Establishment politicians—he beat Ron Paul, the Tea Party’s godfather and tireless exemplar of its passions. And he did so with little jeopardy to his broader appeal.

Romney’s achievement was not only to finish first in Iowa but also to maintain a comparatively moderate presentation. At no point was he compelled, as were other candidates, to propose sweeping, upwardly redistributive transformations of the tax code or radical assaults on government. By resisting the temptation to ostentatiously curry favor with the extreme right, he has preserved his greatest asset for the coming campaign: his electability.

It’s hard to imagine Romney not ending up as the challenger to Barack Obama, even if large portions of the party faithful still yearn for someone else. Santorum’s improbable rise is proof of that, and his comeback ensures the former senator a cycle or two of rapturo...