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.”

In choosing Ms. Butler, who is expected to be sworn in on Wednesday, Mr. Newsom followed through on a pledge he made more than two years ago to name a Black woman to the Senate if a vacancy were to emerge.

But by the time of Ms. Feinstein’s death, three high-profile Democratic members of Congress had already entered the 2024 race to succeed her. Representatives Adam Schiff, Katie Porter and Barbara Lee have all been campaigning statewide in California for months.

Mr. Newsom, who is also a Democrat, told NBC News last month that he preferred not to influence next year’s primary election by giving one candidate an advantage. Instead, he said, he would pick an interim replacement.

His statement drew pushback from political allies of Ms. Lee, 77, who is Black and wants to serve in the office long term. She is currently trailing the other two candidates in fund-raising and polling.

Over the weekend, as speculation gathered around the governor’s choice, members of the Congressional Black Caucus posted a letter to the governor, voicing “strong support for Congresswoman Barbara Lee for that appointment.” Advocacy organizations pushed for Ms. Lee as well.

“Barbara Lee, and Black women, are not mere caretakers, but the voting and organizing center of the national Democratic Party,” Aimee Allison, the founder of She the People, an organization that helps elect women of color in politics, said in a statement last week.

On Sunday night, Ms. Allison said she was pleased with Governor Newsom’s appointment of Ms. Butler and called it “a promise kept.”

Ms. Lee said in a statement that she wished Ms. Butler well and looked forward to working with her on behalf of California. “I am singularly focused on winning my campaign for Senate,” she added.

Mr. Newsom’s chief spokesman, Anthony York, indicated on Sunday that the governor would not demand that his appointee stay out of the 2024 Senate race. In a post on X, the social media site formerly known as Twitter, Mr. York agreed with Representative Cori Bush, Democrat of Missouri, who said “there shouldn’t be any strings attached” to the appointment.

That leaves open the possibility that Ms. Butler, a prolific fund-raiser at Emily’s List, could still enter the primary for the permanent Senate seat.

Ms. Butler, whose appointment was first reported by Politico, did not comment on Sunday night.

She will become the first openly L.G.B.T.Q. senator to represent California. Scott Wiener, a Democratic state senator, celebrated the appointment on Sunday night.

“L.G.B.T.Q. people are being viciously attacked in the United States right now, and elevating a Black lesbian to the U.S. Senate is a powerful statement,” said Mr. Wiener, a gay leader from San Francisco who has championed legislation protecting L.G.B.T.Q. rights.

It is not clear whether Ms. Butler will run to serve a full six-year term. However, her appointment would bring a clear contrast to Senator Feinstein, a wealthy centrist who grew up in an affluent San Francisco family, graduated from Stanford University and prized political compromise throughout her career.

Born in Magnolia, Miss., Ms. Butler, whose father died when she was 16, told The New York Times in 2021 that their household was supported afterward by her mother, who worked, among other jobs, as a security guard, gas station cashier, home-care worker and teaching assistant. Ms. Butler attended a historically Black college, Jackson State University.

Before taking the job at Emily’s List, she spent nearly two decades at the Service Employees International Union, where she organized nurses and janitors before moving in 2009 to California, where she rose to eventually lead the largest union in California, a branch of S.E.I.U. representing 325,000 home-care workers. Her accomplishments in California included the passage of a $15 minimum wage.

Ms. Butler left S.E.I.U. to become a political consultant, working at a firm with some of Mr. Newsom’s top advisers, and was a senior strategist for Ms. Harris when she ran for president. At Emily’s List, she championed diversity, pushing the organization to support nonwhite candidates.

She moved from California to the Washington, D.C., area when she became president of Emily’s List in 2021, and she will re-register as a voter in California, where she still owns a home, according to Mr. Newsom’s administration. Ms. Butler and her wife, Neneki, have a school-aged daughter.

At the time she took the job at Emily’s List, she told The Times that she relished “the opportunity to work for women like my mom.”

“What daughter doesn’t want to continue her mother’s journey?” she said.

Shane Goldmacher, Heather Knight and Reid J. Epstein contributed reporting.