Long expecting to face Mitt Romney in the fall, officials for President Barack Obama's campaign confirmed they have begun looking at Santorum's background for ways to attack his past record.
The former Pennsylvania senator has been riding a wave of momentum after capturing Republican caucuses in Minnesota and Colorado, and a non-binding primary victory in Missouri.
Santorum was in Columbus, Ohio on Saturday, speaking at a Tea Party rally.
"You have an opportunity here in Ohio to put forth a candidate who will present that clear contrast of visions," said Santorum. "Who isn't burdened down by, well let's say a lot of imprudent decisions that were made in their political past."
Santorum also questioned Obama's Christian values, saying the president's agenda is based on "some phony theology. Not a theology based on the Bible. A different theology." He later suggested that the president practices a different kind of Christianity.
The Obama campaign said the comments represent "the latest low in a Republican primary campaign that has been fueled by distortions, ugliness, and searing pessimism and negativity."
Recent polls show Santorum is leading in Ohio, where 63 delegates are at stake.
Romney, the former Massachusetts governor, still leads the delegate race with 123, compared with 72 pledged to Santorum and 32 to former House Speaker Newt Gingrich, according to the most recent AP tally.
The Associated Press contributed to this story.