The Democratic primary for the 2010 United States Senate election in Pennsylvania took place on May 18, 2010, when Congressman Joe Sestak defeated incumbent Arlen Specter, which led to the end of Specter's five-term Senatorial career. Just before the start of the primary campaign, after serving in the Senate as a Republican for 29 years, Specter had switched to the Democratic Party in anticipation of a difficult primary challenge by Pat Toomey; Toomey ultimately defeated Sestak 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.