Arlen Specter, former US Senator from Pennsylvania - Specter was a Democrat from 1951 to 1965, then a Republican from 1965 until 2009, when he switched back to the Democratic Party. He wrote in his 2009 statement, "While I have been comfortable being a Republican, my Party has not defined who I am. ...I have decided to run for re-election in 2010 in the Democratic primary. [but] I will not be an automatic 60th vote for cloture."