Democrats consider Casey, son of former Gov. Bob Casey, the most formidable challenger they could field against Santorum, the Senate's No. 3 Republican.
An independent Quinnipiac survey in mid-February showed Casey ahead of Santorum, 46 percent to 41 percent, among 1,250 Pennsylvania voters surveyed with an error margin of 2.8 percentage points.
The same poll showed Santorum with a 52-31 percent approval rating, with voters saying by almost the same margin that he "deserves to be reelected."
Complicating matters is the likely candidacy of former State Treasurer Barbara Hafer, a Republican-turned-Democrat who disagrees with both Casey and Santorum on the issue of abortion rights. EMILY's List, the pro-abortion rights women's political organization, has already pledged to support Hafer in the Democrat primary over Casey.