Specter was first elected to the Senate as a Republican in 1980, after having served as the Republican district attorney in the Democratic city of Philadelphia. In the Senate, he served as a moderate, socially liberal GOPer. He was challenged from the right for the GOP nomination in 2004 by then-Rep. Pat Toomey. Despite having the full support of the then-popular Bush White House and the GOP establishment, Specter survived the primary by only a 51%-49% margin.
Specter's support among Republicans fell in 2009, after he provided a crucial vote to pass President Obama's stimulus package. Toomey then declared his candidacy for a primary rematch again, and Specter subsequently switched parties when polls showed he would lose the primary. The Obama White House and Pennsylvania Democrats then closed ranks around Specter, but that did not stop Sestak from declaring his candidacy. Specter had big leads in the race until the last few weeks -- but Sestak has been closing fast.