Arlen Specter

Specter changed from Republican to Democrat in 2009 while facing a difficult Republican primary against Pat Toomey. Democratic leaders, including President Obama, supported him, but that backing did not guarantee acceptance by Democratic primary voters. Joe Sestak defeated him in 2010, ending his route to another Senate term. This was a nomination defeat within his new party, not a general-election rejection by his old one, and it shows the limits of transferring an established career between party electorates.