Timing: Santorum is the last of the anti-Romney candidates. And when you are the last to bat, you can be the last to score.
In 2008, John McCain benefitted from peaking at the right time, when he won New Hampshire and Florida, after faltering badly early in the campaign. Santorum never faltered; rather, for a long time, he never really got going. But he is benefiting now from peaking at the right time.
While the national media, pundits, and conservative leaders were flirting with the flavors of the month, Santorum was doing the grueling, unglamorous work of building an organization. That ultimately earned him wins in four of the nine states that have held contests.
Santorum has significant momentum. To maintain it, he will need to prevent Romney from winning both Arizona and Michigan on Feb. 28. He also has to prove that he can win a large, expensive state, and that he can take harder punches from Romney, which Gingrich was unable to do.
More important, he needs to raise $5 million to $10 million over the next two or three weeks to fund his efforts on Super Tuesday, March 6, and in large states later that month and in April.
What Santorum needs most, though, is for Gingrich to exit, which would allow him to consolidate the "anti-Romney" vote.
Not long ago, no one had high hopes for the Santorum campaign - except perhaps Santorum, his family, and his longtime consigliere, John Brabender. Now Mitt Romney's campaign is hoping to come up with an effective response. But how do you beat a candidate who held nearly 400 town-hall meetings and scarcely made a mistake?