She criticized State Rep. Brendan Boyle as having a "mixed position" on abortion, according to Planned Parenthood.
Boyle later responded, saying Margolies was relying on her political connections - she is Chelsea Clinton's mother-in-law - instead of campaigning on the streets.
"This race is not about Bill Clinton. It's about Frank Boyle and the millions like him who need an advocate in Washington, D.C.," Boyle said, referring to his father, who works as a janitor for SEPTA.
Margolies targeted State Sen. Daylin Leach for his attempts to legalize marijuana. She cited a Pottstown teen who was sentenced last week for causing a fatal car crash while under the influence of drugs.
"He was high on marijuana," Margolies said, eliciting laughter and scoffs from the audience.
Earlier, Leach had criticized Margolies for suggesting that the way to fix Social Security was to ask the wealthy "to voluntarily pay more."
Margolies also questioned whether physician Valerie Arkoosh, a first-time candidate, had the political chops to make it in Washington.
Arkoosh earlier had said, "I have shown up for every single debate and taken every opportunity to talk to all of you and let you get to know me." Margolies had declined to participate in several debates before this one.
The candidates seek to fill the largely Democratic Montgomery County and Northeast Philadelphia seat being vacated by Allyson Y. Schwartz, who is running for governor.
The forum was held at Penn State-Abington and moderated by WHYY's Dave Davies.
Boyle, the only candidate who lives in Philadelphia, has strong backing from unions, some of whose members stood in the rain holding signs before the event.
Arkoosh, an anesthesiologist and leading proponent of the Affordable Care Act, had logged the highest fund-raising total as of the last quarter, with most of the funds coming from doctors and medical groups.
Leach, arguably the state's most liberal legislator, touted his passion and ability to pick when to work across the aisle "and when to stay and fight."