(Toomey defeated liberal Sestak in 2010, but 2016 is a presidential-election year and Democrats have a way of turning out lots of voters (whether they're dead, alive or citizens of other countries.)
From the poll:
U.S. Sen. Pat Toomey leads possible Democratic challenger Joe Sestak 45 - 35 percent in an early look at a possible rematch in 2016. Toomey gets a 43 - 25 percent approval rating from Pennsylvania voters, who say 37 - 29 percent that he deserves reelection. His favorability rating is 37 - 21 percent.2016 Presidential Swing State Polls Poll - February 10, 2015 - Florida, Ohio, Pennsylvania Vo | Quinnipiac University Connecticut
For Sestak, 64 percent of voters don't know enough about him to form an opinion.