WASHINGTON—Democratic Sen. Barbara Boxer of California said she won’t run for re-election in 2016, continuing an exodus of longtime liberal lawmakers from Congress.

Ms. Boxer, 74 years old and a vocal Democratic presence in the Senate since 1993, announced her plans in a video interview with her grandson posted Thursday. “I am never going to retire—the work is too important, but I will not be running for the Senate in 2016,” she said.

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