Who is Laphonza Butler, replacing late California Sen. Dianne Feinstein

Laphonza Butler is set to make history this week as the first openly LGBTQ+ person to represent California in the Senate after Gov. Gavin Newsom announced on Sunday that he would appoint her to fill the late Dianne Feinstein's seat.

Butler, currently the president of EMILY's List, a pro-abortion access political group, will be the second openly gay woman to serve in the Senate after Tammy Baldwin was elected in 2012 in Wisconsin.

She will also be only the third Black woman in the chamber and, according to the White House, she will be sworn in on Tuesday by Vice President Kamala Harris -- who was the second Black woman in the Senate.

Harris, a longtime ally of Butler's, will administer her oath in a ceremony at the U.S. Capitol.

"Laphonza Butler has spent her career fighting for the rights of women and working people, just by looking at her career," White House press secretary Karine Jean-Pierre said Monday. "And she's herself succeeding a trailblazer by breaking more barriers."

Feinstein was the first woman senator to represent California.

Equality California, one of the nation's largest statewide LGBTQ+ civil rights organizations, praised the appointment of Butler, calling her "eminently qualified" to represent the state "at a time when our rights and freedoms are under attack across the country."

A longtime Democratic political strategist and labor leader in California, Butler led SEIU Local 2015 -- the largest union in the state representing nursing home and home-care workers -- for more than a decade before leaving in 2019 to become a partner at SCRB Strategies, a political consultancy firm in California now known as Bearstar Strategies, a top advisory group for Newsom and Sen. Alex Padilla, whom Newsom initially tapped to fill Harris' seat after she ran for vice president. (Padilla won a full term in 2022.)

Padilla also celebrated the announcement in a statement on Sunday, saying Butler has been a "strong voice for working families, LGBTQ rights, and a champion for increasing women's representation in politics."

Butler was a senior adviser on Harris' unsuccessful 2020 Democratic presidential campaign before a brief stint at Airbnb, where she served as the director for public policy and campaigns.

In 2021, Butler was tapped to become the president of EMILY's List, the powerful Democratic fundraising group focused on electing pro-abortion access women candidates. She was the first woman of color and first mother to lead the group.

Butler moved to Maryland for the job with her partner and daughter, according to a since-removed bio on the group's website.

Butler's out-of-state residency has become a flashpoint online since Newsom's announcement on Sunday.

The governor's office dismissed the criticism on Sunday night, saying Butler was "a longtime resident and homeowner in California."

Sources tell ABC News that Butler moved in 2021 as part of her role at EMILY's List and that she still owns a home in California and will re-register in the state before being sworn in.

According to Newsom's office, Butler will step down from her role as president of EMILY's List in order to assume her Senate seat.

The U.S. Constitution sets no strict residency requirements to be the senator of a state -- only that a person be a resident of said state upon their election.

In a Monday post to X, formerly known as Twitter, Butler publicly accepted her appointment, calling California the "state I have made my home." She went on to say that she was "honored by [Newsom's] trust in me to serve."

Feinstein, before she died last week, had announced plans to retire when her term ended in January 2025 and the race to succeed her is already underway.

Butler's appointment comes amid an unfolding Democratic primary race in which three high-profile lawmakers -- Reps. Barbara Lee, Katie Porter and Adam Schiff -- are all vying to win the seat that Feinstein was set to vacate. Given California's political tilt, the Democratic nominee will be the heavy favorite to win the general election.

Newsom previously pledged in 2021 to fill any Senate vacancy with a Black woman. But, in an interview last month on NBC's "Meet the Press," Newsom said that the pool of candidates would not include anyone currently running in the Democratic primary -- set for March -- and that the selected person would be an "interim appointment," excluding the possibility of Lee, the only Black woman in the race, from being selected.

That move prompted outcry from Lee at the time, who called the remarks "insulting to countless Black women."

Some of Lee's allies were engaged in a bid over the weekend to convince Newsom to tap the Bay Area congresswoman.

On Sunday, Congressional Black Caucus Chair Steven Horsford sent an open letter to Newsom "strongly urging" him to appoint Lee to the vacant seat.

But following Sunday's announcement, Lee said in a statement on X that she wished Butler well and looked forward to "working closely with her to deliver for the Golden State."

Lee added that she was "singularly focused on winning my campaign for Senate."

It is now an open question whether or not Butler, who has never held public office before, will seek election in 2024. Though Newsom initially billed any appointment he would make as a caretaker, his office clarified their position to The Los Angeles Times on Sunday.

"If that person decides she wants to seek a full term in 2024, then she is free to do so. There is absolutely no litmus test, no promise," Newsom spokesperson Anthony York told paper.

ABC News' Zohreen Shah contributed to this report.