Lisa Friedman, of Silver Spring, Maryland, was selling Clinton T-shirts near the rally close to City Hall when she took up the challenge.
"This woman has been working for more than 50 years to help women and children," she said to the Sanders supporters, immediately drawing boos.
After leaving the stage, someone also shouted at her: "You're supporting corruption! You're part of the problem!"
Many fans of Sanders, a Vermont U.S. senator who competed against Clinton in the party's presidential primaries before endorsing her, feel slighted by the Democratic Party and believe the process was rigged in Clinton's favor.
Friedman, who works for a chain of thrift stores, said she got so excited when Clinton announced her candidacy last year that she designed a T-shirt she has been selling to Democratic gatherings.
"I feel passionate for Hillary," she said. "I don't feel extreme."
Protests around the city drew more than 5,000 people on Monday, the first day of the Democratic National Convention. By the fourth day, crowds had dropped off sharply.
Only a few hundred people gathered for Thursday's rally near City Hall, a big decrease from a day earlier.
By early afternoon, approaching thunderstorms further shrank the demonstrations, sending protesters at the park scrambling to pack up their tents. Police officers drove around blaring warnings that severe storms were coming and anyone outdoors should seek shelter.
Police said they gave out citations carrying fines of $50 each to 103 protesters over the first three days of the convention for code violations including disorderly conduct and obstruction. Seven people were also arrested by the Secret Service for trying to enter secure areas.
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