Butler, who has spent the past two years serving as the president of EMILY’s List, is replacing the late Sen. Dianne Feinstein.

She was sworn in Tuesday by Vice President Harris, becoming the only sitting Black woman senator.

California Gov. Gavin Newsom appointed Butler on Sunday following Feinstein’s death last week at the age of 90. 

The Democrat will not only be the first LGBTQ+ person to represent California in the upper chamber, but she will also be the first ever Black lesbian to serve in the chamber and the third Black woman to serve in the Senate. Vice President Kamala Harris, who previously represented California in the Senate and was the second Black woman to serve in the chamber, will swear Butler in.

Race and Politics Reporter Cheyanne M. Daniels details the historic nature of Butler’s nomination in The Hill.

After Butler’s swearing-in, she will be tasked not only with the daily business of the Senate, but with the decision of whether she should run for reelection in 2024.

Feinstein announced earlier this year that she would not seek reelection, and three high-profile House Democrats have already jumped into the primary fray: Reps. Barbara Lee, Katie Porter, and Adam Schiff.

If Butler does jump into the race, she’ll have some catching up to day – the intraparty battle is already gearing up to be competitive and expensive. NBC News first reported on Monday that Schiff has raised a whopping $6.4 million in the third quarter of this year alone.  

And at least one California Democrat has said Butler should stay out of the Senate race.

“There are three exceptional candidates that have been working more than a year. I just think it’s terribly unfair that she would do so,” said California Rep. John Garamendi told CNN on Monday.