It wasn't clear at first whether voters would take to Casey, the soft-spoken son of a popular former governor. Like Santorum, he is an antiabortion Roman Catholic who opposes gun control. But in contrast to Santorum, Casey is a laid-back moderate who campaigned hard on issues of key concern to the middle class: healthcare and education. He received the endorsement of 18 newspapers across the state, including the Philadelphia Inquirer.

"Casey's lackadaisical style is a strength—voters are tired of polarizing, combative figures," says Terry Madonna, political science professor at Franklin and Marshall College in Lancaster.

Some political observers, however, say that even though a Casey win is likely, a dramatic turnaround for Santorum is not out of the realm of possibility. Jerry Shuster, a University of Pittsburgh professor, warns not to underestimate the power of the GOP turnout machine.

"Democrats historically stay home," says Shuster. "If there's bad weather and if Casey doesn't come out of Philadelphia with a plurality, Casey could be in trouble ... Santorum could win by a point and a half or so."

—Danielle Knight