Rep. Barney Frank (D-Mass.), Chairman of the powerful House Financial Services Committee, will endorse Rep. Joe Sestak‘s (D-Pa.) bid for Senate against a 30-year incumbent.

After Sen. Arlen Specter (Pa.) announced last April that he was switching parties to run as a Democrat for re-election in 2010, the now former Republican immediately had the backing of the national party and the White House. Frank is the first member of Congress to formally endorse Sestak in his uphill primary bid against Specter.

Frank and Lt. Dan Choi, who according to a release from the campaign served in the Army until recently getting discharged until the military’s “Don’t Ask, Don’t Tell” policy, are scheduled to endorse Sestak at separate news conferences Monday in Philadelphia.