Rep. Barbara Lee filed federal paperwork on Wednesday to enter the race for the California Senate seat held by Dianne Feinstein.
Although Lee has not made a formal announcement yet, her entry is widely expected.
Lee filed to create a Senate fundraising committee one day after Feinstein, 89, announced that she would step down from her position after her term ends.
Katie Merrill, a spokesperson for Lee, told The Associated Press that Lee is "filing preparatory paperwork and her announcement will come before the end of the month."
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Lee would join a growing field of candidates, including fellow Democratic Reps. Katie Porter and Adam Schiff, in the contest to fill Feinstein's seat.
Lee said in a statement on Wednesday that it has been an "immense honor" to work alongside Feinstein in California, and highlighted Feinstein's service in Congress.
"I'd like to congratulate Senator Feinstein on her historic, trailblazing career and echo the gratitude of her constituents in our beautiful state for her service," Lee said.
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The 76-year-old is the highest-ranking Black woman appointed to the House Democratic leadership, serving as co-chair of the Policy and Steering Committee.
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Lee is perhaps best known for being the only member of Congress to vote against the authorization for the use of military force after the terrorist attacks of Sept. 11, 2001.
The Associated Press contributed to this report.