Torsella, the former president of the National Constitution Center, announced his decision Thursday night in a statement to supporters and in a video on his Web site.
He said Sen. Specter's recent switch from the Republican to the Democratic party changed the nature of the primaries.
"Now that the dust has settled, it's clear to me that the kind of campaign this would become is not the kind of campaign you or I signed up for," he said. "It would probably be negative, personal, and more about Senator Specter's past than our common future. And that won't do Pennsylvanians any good."
When Specter announced his move to the Democratic party on April 28, he acknowledged that his chances for re-election were stronger with the Democrats. President Obama and other Democratic leaders embraced Specter and have pledged to support him in the 2010 primaries.
Specter brings the Democrats just one seat away from holding a filibuster-proof majority in the Senate--something that could be accomplished if Democrat Al Franken is seated after the protracted legal battle with Republican Norm Coleman in Minnesota.
Torsella entered the 2010 race in February and was considered Specter's main primary challenger. His decision to drop out, however, does not necessarily put Specter in the clear. Rep. Joe Sestak (D-Penn.) has remained critical of the former Republican and has not ruled out his own Senate bid.
Since joining the Democratic party, Specter has shifted his positions on some key issues slightly to the left. He has indicated he is open to supporting a government-sponsored health plan in a health care reform package, and on Thursday, he said the prospects were good that a compromise he could support would be reached over the Employee Free Choice Act, a bill that would make it easier for workers to form unions.