Doris Matsui

At the California Democratic Party convention this year, Representative Doris Matsui, 81, was trying to convince a roomful of delegates that her experience was valuable when a heckler in the back of the room shouted at her.

“Don’t shame young people!” the heckler said.

Ms. Matsui is facing her toughest re-election campaign in her more than two decades of representing a safely Democratic district in the Sacramento region. And the depth of her political vulnerability was evident from the appearance of a powerful supporter who quietly slipped into the room to help her friend: Representative Nancy Pelosi, the former House speaker and the doyenne of California Democrats.

In Tuesday’s primary election, Ms. Matsui is facing an intraparty challenger about half her age at a moment of generational upheaval within the Democratic Party. Her opponent, Mai Vang, 41, a liberal Democrat on the Sacramento City Council, has made a fervent case for generational change, arguing that Democrats need more firepower.

In Sacramento, California’s capital, voters have sent a Matsui to Congress for the last 47 years. Robert Matsui, whose name is etched into the city’s towering federal courthouse, represented the area from 1979 until his death, in January 2005. Two months later, his widow, Doris Matsui, was elected to fill his seat. She has won with ease ever since, championing flood protection for a city at the confluence of two rivers.

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