Arlen Specter (D) reelected in 2010

Even if he won the primary (he would have to pick up 60,000 votes to overcome Joe Sestak), then he probably loses even more than Sestak did in the general election. There were a lot of headwinds against the Democrats in 2010.
 
If he pulls off the win:

Assuming he still passes away from his NHL the governor at the time was Tom Corbett who would appoint his replacement. There was a pretty sizable bench of Pennsylvania Republicans to draw upon including conservatives and moderates like Mike Fitzpatrick. The special election would probably favor the republican without the presidential level turnout of Democratic voters, though it's possible it's someone like Tim Murphy and the Democrats reclaim the seat (in which case they likely hold it in 2016). If the seat is held by the Republicans Sestak might be the 2016 nominee (not being seen as a past loser) and could capture it.

Hard to get into it without covering current politics, but in that case you'd add a +1 to the Democratic senate numbers (and consider any close votes changeable) and look at the Pennsylvania senator as having a possible place on a future ticket.
 
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