For much of the election year, Santorum, the third-ranking member of the GOP leadership, has been one of the most embattled Republican senators, often trailing Casey by double digits in the polls. Santorum tried to position himself as strong on national security, but his staunch support of Bush and the Iraq war as well as his conservative firebrand style led to low approval ratings.

Like Santorum, Casey is an antiabortion Roman Catholic who opposes gun control and withdrawing troops from Iraq. But Casey called for more accountability in Iraq and campaigned hard on issues of key concern to the middle class: healthcare and education. Jennifer Duffy, a Senate election analyst with the Cook Political Report, says the Casey campaign was "brilliant" by remaining low key, not holding many debates, and not taking well-defined positions on some issues. "Santorum," she says, "was never really able to draw sharp contrasts with Casey."

The race was one of the costliest in Pennsylvania's history. But the $22 million raised by Santorum (compared with Casey's $16 million) was still not enough. Santorum's loss is expected to open up a battle in the Senate for Republican whip, a post Santorum was likely to fill. Possible contenders include Sens. Trent Lott of Mississippi and Lamar Alexander of Tennessee.