Santorum vs 0bama would be a replay of Santorum vs Bob Casey...
Casey is the son of a popular pro-life governor who had recently passed away before the election. He also "claims" to be pro-life, but has proven to be a Harry Reid yes man. Ask him, right now, where he stands on Odumbo's edict to the Catholic Church. Also, conservatives punished Santorum for supporting Specter over Toomey (I voted for Santorum, but I was angry for his position during the Toomey primary).
Odumbo is nowhere near as popular as Casey was during that election, and, as it stands now, Santorum is the most Conservative of the remaining candidates (I do not consider Ron Paul a Conservative).