Torsella said he filed paperwork Monday to form a campaign committee so he can raise and spend money. He said he has been raising money, hiring staff and traveling the state in advance of the formal announcement of his candidacy in the weeks ahead.
Several other prospective candidates are considering running for the Democratic nomination to take on Specter, who has said repeatedly he plans to seek a sixth six-year term.
MSNBC political commentator Chris Matthews flirted with the idea, but said last month that he had decided to remain in television.
Torsella has never held elected office. He served as a deputy to now-Gov. Ed Rendell when he was Philadelphia mayor in the 1990s and led the Constitution Center, a nonprofit museum near Independence Hall, from a concept in 1996 to the opening of its doors in 2003.
Torsella quit the museum to run for a suburban Philadelphia congressional seat in 2004, but lost the Democratic primary to Rep. Allyson Schwartz. He returned to the center in 2006 for what was described as a transitional tenure that recently ended.
Specter, a political moderate, is longest-serving senator in Pennsylvania's history. He narrowly fended off a 2004 primary challenge from then-U.S. Rep. Pat Toomey, who now heads the conservative Club for Growth in Washington.
The senator, who turns 79 on Thursday, has survived bouts with cancer during his tenure, including treatment last year for a recurrence of Hodgkin's disease, but he maintains a busy schedule that includes daily games of squash.
No challenger has emerged from within the GOP so far.
Toomey has said that he is considering a possible run for governor in 2010.
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