When Sen. Arlen Specter (D-PA) announced that he would switch parties and run for re-election as a Democrat he all but forced Joe Torsella–former Deputy Mayor of Philadelphia–out of the race. Were it not for the likely candidacy of Rep. Joe Sestak (D-PA), that would have cleared the field for Specter, who would almost certainly have sailed to the Democratic nomination.
This is a companion discussion topic for the original entry at https://talkingpointsmemo.com/?p=141211