Specter is a liberal. I have no idea why he ever wanted to add Repub to his name OTHER THAN conduct behind the scenes espionage and treachery against the conservatives.
Specter became a "Republican" in 1965 because there was no room for him to seek the Philadelphia district attorney's position with the majority Democrats. The Democrats have owned Philadelphia since 1951, but they also support the popular Arlen Specter brand of liberalism there.