WASHINGTON — Representative Adam B. Schiff, the California Democrat who emerged as one of President Donald J. Trump’s chief congressional tormentors from his perch atop the House Intelligence Committee, announced on Thursday that he would seek the Senate seat long held by Dianne Feinstein.
“I wish I could say the threat of MAGA extremists is over,” he said in a video on Twitter. “It is not. Today’s Republican Party is gutting the middle class, threatening our democracy. They aren’t going to stop. We have to stop them.”
Mr. Schiff, 62, is the second member of California’s Democratic congressional delegation to join the 2024 race, after Representative Katie Porter. He enters the campaign with the largest national profile, built from his position as the manager of Mr. Trump’s first impeachment trial. He later served on the House committee responsible for investigating the origins of the Jan. 6, 2021, attack on the Capitol.
On Tuesday, Speaker Kevin McCarthy, a Republican, exiled Mr. Schiff and Representative Eric Swalwell, another Democrat, from the House Intelligence Committee in retribution for their actions toward Republicans when Democrats held the majority.
Ms. Feinstein, 89, has not said whether she will run again in 2024 but is widely expected not to do so as she faces Democratic worries about her age and ability to serve. Last year, she declined to serve as president pro tem of the Senate, and in 2020 she ceded her post as the top Democrat on the Senate Judiciary Committee after coming under pressure from her party during the Supreme Court confirmation hearing of Justice Amy Coney Barrett.
In an interview with The New York Times on Thursday, Mr. Schiff said he had first apprised Ms. Feinstein of his plans several weeks ago, in person, on Capitol Hill and again by phone on Wednesday.
“She was very gracious,” he said. “I let her know that I wanted to make my announcement, and she could not have been nicer about it.”
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Mr. Schiff said that he did not want to speculate about whether Ms. Feinstein might retire, and that she deserved to set her own schedule for making an announcement about her political future.
“Once more, I have a genuine admiration and affection for her, and wanted to do everything I can to respect that,” he said.
A former federal prosecutor, Mr. Schiff served in California’s State Senate before being elected to a Los Angeles-area House seat in 2000.
In Congress, he became a close ally of Speaker Nancy Pelosi, who tapped him to play a leading role in Mr. Trump’s impeachment trial and then on the Jan. 6 committee. Last fall, Mr. Schiff passed on a chance for a slot on the post-Pelosi House leadership team to focus on a planned run for the Senate.
During and after the Trump years, Mr. Schiff became one of the most prodigious fund-raisers in Congress. During the 2018 election cycle, he raised $6.3 million, and then his fund-raising surged to $19.6 million in 2020 and $24.5 million in 2022 — without a competitive election of his own to wage. He has not faced a serious challenge since arriving in Congress, winning each of his general elections by at least 29 percentage points.
According to the latest Federal Election Commission reports, Mr. Schiff had $20.6 million in campaign money at the end of November, compared with $7.7 million for Ms. Porter and $54,940 for Representative Barbara Lee, who has told donors of her plans to run.
While Mr. Schiff and Ms. Lee’s House seats are safely Democratic, Ms. Porter’s is far more contested; she won re-election in November by three percentage points.
Other California Democrats considering running for Senate include Representative Ro Khanna of Silicon Valley. Because of California’s all-party primary system, the election is not likely to feature a competitive Republican candidate; in 2016 and 2018, two Democrats advanced to the Senate general election. Last year, a Republican won 39 percent of the vote against Senator Alex Padilla, who had been appointed to finish Vice President Kamala Harris’s term.
On the prospect of facing more than one House colleague in the race, Mr. Schiff described fellow Democrats as “rivals under the same flag.” Still, he said his experience as an impeachment manager and as a member of the Jan. 6 committee stood out, along with his efforts to make health care more affordable and raise wages.
California, the nation’s most populous state with nearly 40 million residents, has not hosted a highly competitive contest for an open Senate seat since 1992, when Ms. Feinstein and Barbara Boxer, a fellow Democrat, were both elected for the first time.
Ms. Feinstein, who is in her sixth term, has been trailed by questions about her fitness to serve. Problems with her short-term memory have become an open secret on Capitol Hill, though few Democrats have been willing to discuss the subject publicly.
She has made no moves to suggest she will seek re-election in 2024. She has not hired a campaign staff and, in the latest campaign finance report for the period ending in September, had less than $10,000 in cash on hand, a paltry sum for a sitting senator.
Not since the early 1990s have both sitting senators from California been men. When asked whether electing a woman might be a priority for some voters after the Supreme Court’s repeal last year of Roe v. Wade, Mr. Schiff played down the potential role of gender in the race.
“I’m very proud of my fierce efforts to protect women’s reproductive freedom and my pro-choice record is a stellar one,” he said.
Mr. Schiff had earlier suggested that his election to the Senate could be symbolic in another way.
“I think a lot of Californians will relish the idea of making Adam Schiff Kevin McCarthy’s home-state senator,” he said.