The most dramatic moment came when a man in a black T-shirt rose and shouted angrily at Sestak.
"I'm a veteran, and I should be heard!" he shouted, and turned to walk out of the room.
"Chris, is that you?" Sestak said.
"Yeah," the man said, turning to face Ses-tak.
"There's a wonderful biker VFW post that I often go to, and Chris and I sometimes have disagreements," Sestak explained to the audience. "But he's actually got my cell phone and called me today."
Sestak promised the man, who later identified himself to reporters as Chris Hill, that he'd get the next question.
When Hill got a floor microphone, he told Sestak that he'd read the House version of the health-care bill and didn't see how it would keep his employer from dumping the private insurance he has now to save money.
"How can you tell me the private option will stay in place," Hill shouted, "when my employer with 29,000 employees can just turn around tomorrow and say 'We're writing that off the bottom line; you guys are all going to the public option?' "
Sestak said that the bill "mandates that the employer has to keep the health-care plan he has with you."
An aide later explained that the bill actually requires large employers either to provide health coverage or pay a tax, which congressional analysts have concluded will result in most employers' maintaining their plans.
Some conservatives grumbled that Sestak had ensured that he'd have a friendly crowd by holding the event not in his suburban district but in Philadelphia.
Amy Ringenbach, a coordinator for the activist group MoveOn.org, said that her organization and others had waged an energetic Internet-based effort to turn out supporters, because other congressional town halls had been disrupted by opponents of the health-care bills.
Sestak, who appeared in shirtsleeves on stage, was low-key and at times wonkish, patiently explaining details of co-pays, mandates and cost incentives.
He never mentioned Specter, whom Sestak is challenging in the Democratic primary next year.