Laphonza Butler's wife Neneki Lee holds the Bible as Vice President Kamala Harris  swears in Laphonza Butler during a re-enactment of the swearing-in ceremony.

Laphonza Butler is sworn in, filling Dianne Feinstein’s Senate seat

WASHINGTON — Laphonza Butler was sworn in Tuesday to fill the seat of the late Sen. Dianne Feinstein, D-Calif., who died last week at the age of 90.

Vice President Kamala Harris, who has been a longtime ally and adviser of Butler, administered the oath of office, prompting boisterous applause from the Senate floor and gallery.

California Gov. Gavin Newsom moved quickly to select Butler, who was the president of EMILY’s List, on Sunday. She is the third Black woman to serve as a senator, behind Harris and Sen. Carol Moseley Braun, D-Ill., in the 1990s.

The first openly LGBTQ person to represent California in the chamber, she will serve alongside fellow California Democrat Sen. Alex Padilla.

After her swearing-in ceremony, President Joe Biden called Butler to congratulate her, the White House said.

Laphonza Butler smiles during the ceremony.With Butler’s swearing-in, the Democrats again have a 51-49 majority. Stephanie Scarbrough / AP

Butler had led EMILY’s List, a group focused on electing Democratic women who support abortion rights, since 2021, when she became the first Black woman to lead the organization.

“I am honored to accept Gov. Newsom’s nomination to be a U.S. Senator for a state I have long called home,” Butler said in a statement Monday. “I am humbled by the Governor’s trust. Sen. Dianne Feinstein’s leadership and legacy are immeasurable. I will do my best to honor her by devoting my time and energy to serving the people of California and the people of this great nation.”

Butler resides in Maryland, but a spokesman, Matthew Wing, told NBC News on Monday that she had re-registered to vote in California.

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With her swearing-in, the Democrats again have a 51-49 majority; it puts them in a position to fill Feinstein’s seat on the Senate Judiciary Committee, which is currently deadlocked.

Feinstein had said she would not run for re-election in 2024, and major contenders in the Senate race are Democratic Reps. Adam Schiff, Katie Porter and Barbara Lee.

Before Feinstein’s death, Newsom had told NBC News that he planned to make an “interim“ appointment if he needed to fill her seat, because he didn’t want to tip the scales toward any of the current candidates.

He made it clear, however, that there were no restrictions put on Butler’s appointment and that she was free to run for the Senate if she wanted. Butler will serve out the remainder of Feinstein’s term, which ends in early 2025. She has not yet indicated whether she plans to run for the Senate seat.

Rebecca Shabad

Rebecca Shabad is a politics reporter for NBC News based in Washington.

Liz Brown-Kaiser

Liz Brown-Kaiser covers Capitol Hill for NBC News.

Adam Edelman and Frank Thorp Vcontributed.