So the question now is simple: how serious an opponent for Mitt Romney can Rick Santorum be?

Santorum leaves Iowa with momentum, and he will have some money now. I get the sense also that even if Newt Gingrich and Rick Perry stay in and try to compete in South Carolina and Florida, their numbers will drop, and a good chunk of that support will probably move to Santorum rather than to Romney. Gingrich, in his concession speech, effectively endorsed Santorum.

Now it will become in essence a two-person race. That’s a totally new environment. How does each behave in it? The Romney people had no reason to suspect until recently that they’d even have to think about Santorum. How are they planning on trying to attack him? Romney, remember, did a great job of finding Rick Perry’s Achilles’ heel: that Texas law providing in-state tuition for children of illegal immigrants. So he can flash the blade when he has to. Santorum, meanwhile, we’ve never seen in the pressure cooker. How will he respond? How will he choose to attack Romney? For the past three weeks or so, somehow the Romney-as-moderate-sellout narrative has receded into the background a bit, as Ron Paul and then Santorum rose to first-tier status. Let’s see if Santorum can bring these points back into the conversation. He had what struck me as a pretty good line Tuesday morning on TV: “We cannot put up a presidential candidate who is in basically the same place as Obama on government-run health care.”

It’s hard to imagine that Santorum can ultimately gather up the disparate elements, the Paulites and the Newties and other disaffecteds, aggregate them into one force, and challenge Romney in a serious way. But this is his moment in the sun. Please forgive the sports analogy, as I try to avoid them, but it’s like the backup quarterback finally getting his long-dreamed-of start. Some of them rise to the occasion, some don’t. If Santorum has that rising in him, this can be interesting for a while yet, because we still know that no one exactly loves Romney.