Sen. Arlen Specter (D-Pa.) earned a key endorsement from the Pennsylvania chapter of AFL-CIO in his bid for a sixth term.

The union was a key backer for Specter during his 2004 reelection campaign, when he was still a Republican. On Tuesday, it again backed him over a primary opponent -- Democratic Rep. Joe Sestak (D-Pa.).

Specter tweeted Tuesday from his verified account @SenArlenSpecter:

"Thank you, thank you, thank you, PA AFL-CIO. I'm proud to have your endorsement and support."

The former Republican has already received the endorsement of the Pennsylvania Democratic Party.

Poll numbers show the incumbent leading by a wide margin in the May 18 primary. Sestak has failed to build name recognition across the state at this point, but he had about $5 million in the bank at the end of the year.

Former Rep. Pat Toomey (R), the likely Republican nominee, is running neck-and-neck with Specter in the general election.

Cross-posted to the Twitter Room