Katie Porter, California Congresswoman, Is Running for Senate

WASHINGTON — Representative Katie Porter, a third-term California Democrat who studied under Elizabeth Warren at Harvard University and became a social media darling of liberal Democrats, said Tuesday that she would run in 2024 for the Senate seat held by Dianne Feinstein.

Ms. Porter, 49, is the first announced challenger to Ms. Feinstein, 89, who has not declared her intentions about 2024 but is widely expected to not seek re-election amid Democratic worries about her age and ability to serve. Last year, Ms. Feinstein declined to serve as president pro tem of the Senate and earlier relinquished her post as the top Democrat on the Senate Judiciary Committee under immense pressure after the Supreme Court confirmation hearing of Justice Amy Coney Barrett.

As Ms. Warren did four years ago, when she was the first major Democrat to enter the 2020 presidential contest, Ms. Porter jump-starts a race that is certain to be among the most expensive intraparty contests in the country. A vaunted fund-raiser, Ms. Porter became widely known for her combative treatment of witnesses from the financial sector and Trump administration officials who appeared before her on the House Oversight Committee.

The Iowa-born Ms. Porter was a leading surrogate for Ms. Warren’s 2020 campaign and often hosted small events promoting her mentor. She worked as a law professor at the University of California, Irvine, and in 2012 was appointed by Kamala Harris, then the California attorney general, to oversee a $9 billion settlement after the mortgage crisis. She was elected to Congress in 2018.

A competitive California Senate campaign is certain to be among the most expensive races in the country in 2024.

Other California Democrats believed to be considering bids for Ms. Feinstein’s Senate seat include Representative Adam Schiff, who has already hired staff members in preparation for a statewide campaign, and Representative Barbara Lee, who has told donors of her plans to run. Neither has formally announced a campaign.

Ms. Feinstein, in her sixth term, has been dogged by questions about her fitness to serve. Issues with her short-term memory have become an open secret on Capitol Hill, though few Democrats have been willing to discuss the subject publicly.

Aides to Ms. Feinstein did not immediately respond to Ms. Porter’s announcement on Tuesday.