2010 United States Senate Democratic primary election in Pennsylvania

 In the United States Senate Democratic primary election in Pennsylvania on May 18, 2010, Congressman Joe Sestak (pictured) defeated incumbent Arlen Specter, who had been in the Senate for five terms as a Republican. Just before the primary campaign, Specter switched to the Democratic Party in anticipation of a difficult primary challenge by Pat Toomey. Sestak was ultimately defeated by Toomey in the general election. Political observers and journalists described the race between Specter and Sestak as one of the bitterest and most watched of all the 2010 primary elections. Specter led Sestak by more than 20 percentage points for most of the race, but this lead narrowed in the final month of the campaign, when Sestak concentrated his funds and efforts on television commercials questioning Specter's Democratic credentials. Political observers said that Sestak's commercials and a national swing in momentum against incumbents harmed Specter's chances.