California Gov. Gavin Newsom on Sunday selected Laphonza Butler, a Democratic strategist and adviser to Kamala Harris’ 2020 presidential campaign, to fill the vacant US Senate seat held by the late Sen. Dianne Feinstein.

In choosing Butler, Newsom fulfilled his pledge to appoint a Black woman if Feinstein’s seat became open. However, he had been facing pressure from some Black politicians and advocacy groups to select Rep. Barbara Lee, a prominent Black congresswoman who is already running for the seat.

Butler will be the only Black woman serving in the US Senate, and the first openly LGBTQ+ person to represent California in the chamber.

Newsom said in a statement that the priorities Feinstein fought for in Congress — reproductive freedom, equal protection and safety from gun violence — were under assault in the nation.

"Laphonza will carry the baton left by Sen. Feinstein (and) continue to break glass ceilings and fight for all Californians in Washington D.C.,” he said.

Butler leads Emily’s List, a political organisation that supports Democratic women candidates who favour abortion rights. She also is a former labour leader with SEIU 2015, a powerful force in California politics.

Her appointment sets up a potentially tricky political calculus in the crowded 2024 contest to succeed Feinstein, which has been underway since the beginning of the year.

Newsom spokesman Anthony York said the governor did not ask Butler to commit to staying out of the race. The deadline for candidates to submit paperwork to seek the office is December 8. Should Butler enter the contest, she could set up a competition for the relatively small but influential group of Black voters in California and possibly undercut Lee's chances.

Emily's List is known as a fundraising powerhouse, and raising huge sums of campaign cash is a must in any statewide California race. Newsom's statement said she will step down from the organisation.

The decision carried the threat of political fallout for Newsom, who is seen as a potential future national candidate. The candidate favoured by Black voters has won the Democratic Party’s presidential nomination every cycle since 1992. The Congressional Black Caucus was among the groups and Black politicians that had urged Newsom to appoint Lee, calling her the best-qualified choice for the post.

The long-serving Feinstein died last Thursday at age 90 after a series of illnesses.

Butler currently lives in Maryland, according to her Emily's List biography. Izzy Gardon, a spokesman for Newsom, said Butler owns a home in California. She is expected to reregister to vote in California before being sworn in. That could happen as early as Tuesday evening when the Senate returns to session.

Butler did not immediately respond to an email seeking comment nor release a statement.

Democrats control the Senate 51-49, though Feinstein’s seat is vacant. A quick appointment by Newsom will give the Democratic caucus more wiggle room on close votes, including nominations that Republicans uniformly oppose.

Feinstein, the oldest member of Congress and the longest-serving woman in the Senate said in February she would not seek reelection in 2024. Lee is one of several prominent Democrats competing for the seat, including Democratic U.S. Reps. Katie Porter and Adam Schiff. Newsom said he did not want to appoint any of the candidates because it would give them an unfair advantage in the race.

Butler has never held elected office but has a long track record in California politics. She served as a senior adviser to Harris's 2020 presidential campaign while working at a political firm filled with strategists who have worked for Newsom and many other prominent state Democrats. She also briefly worked in the private sector for Airbnb.

She called Feinstein “a legendary figure for women in politics and around the country,” in a statement posted after Feinstein's death.

Emily's List, the group Butler leads, focuses on electing Democratic women who support abortion rights. With the US Supreme Court's 2022 decision to overturn women's constitutional right to abortion, the issue has become a galvanising one for many Democrats.

Equality California, the nation’s largest statewide LGBTQ+ civil rights organisation, said the appointment “will give our ... community another voice in Congress at a time when our rights and freedoms are under attack across the country.”

It’s not Newsom’s first time selecting a U.S. senator, after being tasked with choosing a replacement for Kamala Harris when she was elected vice president; at that time he selected California Secretary of State Alex Padilla for the post. It was one of a string of appointments Newsom made in late 2020 and early 2021, a power that gave him kingmaker status among the state’s ambitious Democrats.

The seat is expected to stay in Democratic hands in the 2024 election. Democrats in the liberal-leaning state have not lost a statewide election since 2006, and the party holds a nearly 2-to-1 voter registration advantage over Republicans.