[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2009/images/05/14/art.bospecter0514.gi.jpg caption="The withdrawal of Joe Torsella is good news for new Democrat Sen. Arlen Specter of Pennsylvania, pictured here with the president soon after switching parties."]
WASHINGTON (CNN) - Pennsylvania Sen. Arlen Specter received welcome news Thursday evening when his only current primary challenger abandoned a run for the Democratic Senate nomination, two Democratic sources tell CNN.
Joe Torsella, the former president of the National Constitution Center, will officially announce his decision on his campaign Web site Thursday evening.
Specter and national Democratic leaders are still waiting word on whether the former Republican will be challenged for the Democratic nomination by Rep. Joe Sestak, who has not ruled out a bid for the Senate nomination.
In several interviews since Specter's switch, Sestak has been critical of the senator.
"I'm not sure he's a Democrat yet," the congressman told CNN's John King on State of the Union nearly two weeks ago.
Specter recently renounced his GOP affiliation and joined the Democratic Party. His switch is key to helping give President Obama a filibuster-proof majority - in theory - in the Senate.
Specter gives Democrats 59 votes in the chamber. Democrat Al Franken would give Democrats the 60th vote needed once his Senate race with GOP Sen. Norm Coleman is resolved. It is believed that Franken, who holds a lead of a few hundred votes over Coleman, will prevail in the courts.
Obama and national Democratic leaders embraced Specter after he switched parties, and pledged to support him in his 2010 re-election bid.
UPDATE 9:33 pm.: Joe Torsella makes it official.
(See Torsella's statement and link to his video announcement after the jump.)
STATEMENT OF JOE TORSELLA ON HIS CAMPAIGN FOR US SENATE
Philadelphia, PA – Today Joe Torsella sent the following message to his friends and supporters regarding his campaign for the United States Senate.
You can watch Torsella's video announcement regarding his campaign at:
[the video is available on YouTube]
"As you know, earlier this year I decided to run for the United States Senate from Pennsylvania. I thought we needed a Democratic Senator who would champion new ideas and new solutions to old problems.
"But as we’ve all learned, campaigns can be unpredictable.
"Senator Specter’s decision to join the Democratic Party transformed this election.
"Over time, it’s become clear to me that the kind of campaign this would become is not the kind of campaign I – or you – signed up for. It would probably become negative, personal, and all about Senator Specter’s past, not our common future. And that doesn’t do Pennsylvania any good.
"So I’ve decided that I’m not going forward with my campaign for the Senate under these circumstances.
"I’ve posted a video on our campaign website explaining my decision and I hope you’ll take a moment to view it.
"If you were kind enough to contribute, we’re going to tally up the accounts and send your money back. I know you wanted to send me to the Senate, not to send me a political blank check.
"And I want you to know, I’m not going anywhere. I intend to play an active part in the conversation about renewing America that has started in Washington, but is far from finished.
"There will be other opportunities to serve the country and state we love and to draw America just a little closer in our generation to the bright promise of our founding ideals. I hope you and I will both be part of that, together.
–CNN's Martina Stewart contributed to this report.