Gov. Gavin Newsom of California announced on Sunday night that he has chosen Laphonza Butler, the president of Emily’s List and a former labor leader, to fill the vacancy left in the Senate by the death of Dianne Feinstein.

Ms. Butler, 44, has been a fixture in California politics for nearly 15 years as a former leader of the state’s largest labor union and an adviser to Vice President Kamala Harris. In 2021, she became the first Black woman to take the helm at Emily’s List, the fund-raising powerhouse dedicated to electing female candidates and supporters of reproductive rights.

Senator Feinstein died at 90 on Thursday in Washington, D.C., and her body was flown over the weekend back to San Francisco, where a memorial service has been scheduled for this Thursday.

“As we mourn the enormous loss of Senator Feinstein, the very freedoms she fought for — reproductive freedom, equal protection and safety from gun violence — have never been under greater assault,” Mr. Newsom said in a statement. “Laphonza will carry the baton left by Senator Feinstein, continue to break glass ceilings and fight for all Californians in Washington, D.C.”