Liberal Democratic Rep. Barney Frank of Massachusetts endorsed Rep. Joe Sestak's primary challenge to Sen. Arlen Specter yesterday, becoming the first member of Congress to back the insurgent Pennsylvania candidate.

Specter, a 28-year senator who became a Democrat in April, has the support of President Obama, Gov. Rendell, and national and state party leaders in his bid for a sixth term.

Frank, chairman of the House Financial Services Committee, called Sestak a "true Democrat" and said he valued Sestak's leadership on economic and military issues.

Frank, who is openly gay, also said he admired Sestak's commitment to ending the military's don't-ask-don't-tell policy against gay people serving openly and to repealing the Defense of Marriage Act, which allows states to refuse to recognize same-sex marriages performed in other states.

Specter also opposes don't-ask-don't-tell and recently called for repeal of the marriage act, which he voted for when it was enacted in 1996.

Nominating Sestak is "important for the integrity of the political system," Frank said during a news conference at the Center City law firm of Kohn, Swift & Graf.

"Sen. Specter has had a very distinguished political career, but he made it clear that he had left the party he had been in for a long time solely because he didn't think he could be reelected in that party, and he changed his views on a number of issues."

It's unclear whether Frank's endorsement will carry much weight with Pennsylvania primary voters, but given his stature as a giant of the liberal wing of the Democratic Party, his move might encourage others to break with the White House.

Frank is also a hero in the gay community and may help Sestak with fund-raising ties. The endorsement comes as liberal bloggers have been attacking Sestak for supporting Obama's plan to send 30,000 more troops to Afghanistan. Frank opposes the policy.

Contact staff writer Thomas Fitzgerald at 215-854-2718 or tfitzgerald@phillynews.com.