Updated: 11:57 p.m.

Former National Constitution Center President Joe Torsella (D) dropped out of the Pennsylvania Senate race Thursday evening, leaving Sen. Arlen Specter as the only announced Democratic candidate in the race at the moment.

However, Rep. Joe Sestak (D) is continuing to seriously consider challenging Specter in the 2010 Democratic primary, after Specter announced his party switch two weeks ago. Sestak could get the support of organized labor if he runs.

Torsella announced Thursday in a Web video that he was dropping his campaign and that he would refund all contributions to his supporters. He stressed that he had not been pressured to drop his campaign.

“No one asked me to take this step, and I haven’t asked for, or been offered anything, to do it,” Torsella said in his announcement. “Just like my decision to run in the first place, it simply feels to me and my family like the right thing to do.”

Both the White House and the Democratic Senatorial Campaign Committee backed Specter after he announced he was switching parties, leaving Torsella to run what would appear to be a quixotic campaign against the president and Senate leaders.

“In the last few weeks it’s become clear that the Democratic primary is more likely to be decided by questions of partisanship, politics, and personal issues, most of which will probably be negative,— said a source close to Torsella, before his withdrawal was official. “That’s not what Joe signed up for. He has chosen to not run under those conditions and will look for another opportunity to serve Pennsylvania and the country.—

In his video to supporters, Torsella acknowledged that campaigns can be unpredictable and that Specter’s switch had transformed the election. He also pledged that he will continue to stay involved in Democratic Party politics.

Torsella is best known for building the National Constitution Center in Philadelphia, a project that Specter aided over the course of its execution. The two men have shared a good personal relationship over the years. Torsella also lost the Democratic nomination to now-Rep. Allyson Schwartz in an open-seat race in 2004.