Rep. Barney Frank (D-MA) has decided to endorse Rep. Joe Sestak (D-PA) over Senator Arlen Specter (D-PA) for the 2010 senatorial race in the Keystone State. Frank is the first member of Congress to back Sestak for the Democratic primary.

That news must sting for Specter, a thirty-year incumbent. He’s spent the year flip-flopping on the issues in a desperate attempt to paint himself as a reliable liberal Democrat. He switched parties after polls showed him losing to conservative frontrunner Pat Toomey, a former House member.