The most recent Rasmussen poll has Santorum up 1 point over Obama.

A big gap closed up suddenly:

Rasmussen Reports, 2/2-2/3: Obama 44, Santorum 45
Rasmussen Reports, 1/31-2/1: Obama 46, Santorum 44
Rasmussen Reports, 1/23-1/24: Obama 48, Santorum 40
Rasmussen Reports, 1/17-1/18: Obama 48, Santorum 38

ADDED: By the way, there's a lot of talk about how Rush Limbaugh hasn't endorsed anyone, but — based on my own listening to nearly every recent show in the last month — I think he's been subtly influencing listeners to choose Santorum. Limbaugh frequently talks about how he doesn't do endorsements, that his "candidate" of choice is conservatism, and that he wants to be in a position to support whoever goes up against Obama in the fall. But he has repeatedly pointed to Santorum as the most conservative candidate and, perhaps more important, the candidate who will be most able to articulate conservatism when confronted with Obama.