"We're overdue for a real debate on the issues, priorities and leadership voters want from their senator," De León said in an interview. "I think California needs a senator not just fully resistant to Trump's presidency, but who understands the issues most Californians face every day."
De León announced his bid in an email to supporters and a video, where he painted a dire picture.
"We now stand at the front lines of a historic struggle for the very soul of America, against a president without one," he said. "Every day, his administration wages war on our people and our progress. He disregards our voices. Demonizes our diversity. Attacks our civil rights, our clean air, our health access and our public safety."
De León (D-Los Angeles) said he will focus on improving quality of life, increasing educational opportunities, cleaning the environment and creating universal healthcare.
"I am running for the U.S. Senate because you deserve a seat at the table...," he said. "To achieve these goals, expand the California dream, and take the fight to Trump from California to Washington, D.C., I commit to working tirelessly to earn your vote here at home, and once elected, to do my part to work even harder to reunite this nation with a progressive agenda."
The announcement sets the stage for a bitter intraparty battle next year, pitting Feinstein, who epitomizes the Democratic old guard, against a member of the party's ambitious younger generation seeking to climb the political ladder. It also presages a costly and divisive fight at a time many California Democrats argue their energy and dollars would be better spent on several congressional races in the state that could determine who controls Congress.
Bill Carrick, Feinstein's longtime political advisor, described De León's bid as "wasting money and energy on what will turn out to be a rather difficult campaign for Sen. De León.… He's a virtual unknown. He's a termed-out politician looking for a gig."
Carrick predicted Feinstein would do "very, very well" in the contest, noting her strength with female voters, her base in Northern California and her history of winning Los Angeles County.
"If he sees an opening, it's a mirage," Carrick said of De León.
Feinstein, who toured fire damage in Northern California with Gov. Jerry Brown and fellow Sen.
Earlier, Feinstein seemed unfazed about the prospect in an interview with The Times.
"I am what I am; I'm pretty well known, and people, I assume, will come after me any way they can. That's up to them," Feinstein said. "If that's of any value to people, I'll win; if it's not, I won't."
She made the remarks as rumors about De León challenging Feinstein — who, at 84, is the oldest member of the Senate — crescendoed.
Unlike previous years, she has faced heated criticism of late from liberal critics who have said her measured approach is no longer representative of a state that has become home to "the resistance" to President Trump and his policies. Feinstein also drew rebukes from members of her party when she called for "patience" with Trump this summer, saying he could develop into a good president.
De León faces significant challenges in his effort to unseat Feinstein. She is a well-respected party elder who is among the most powerful Democratic forces in the state and the nation. When then-Sens. Barack Obama and Hillary Clinton sought a detente during the 2008 election, it was hosted in her living room. She is also among the wealthiest members of Congress and could easily self-fund a campaign if she needed to. And since she was first elected to the chamber in 1992, she has earned seniority and bipartisan trust in a chamber where both are critical, especially for members of the party out of power.
De León said he is used to taking on tough battles.
"We're taking on the establishment, there is no doubt," he said. "But I've taken on the establishment all my life, and I've been told to wait my turn.… Now is the time for change and I look forward to having this debate of ideas, of vision for the state."
Still, he seemed cognizant that he must tread carefully. When asked about policy differences, he said he wanted to be "very respectful" toward Feinstein, before pointing to the use of the military overseas.
"Sen. Feinstein is an aggressive hawk on foreign policy matters and military intervention and a conservative incrementalist on domestic issues," he said, adding that he would take the opposite approach.
The biggest endorsement De León received following his announcement was from Democracy for America, the progressive political action committee formed by former Vermont Gov. Howard Dean in 2004. He was also backed by Assembly members Lorena Gonzalez Fletcher of San Diego and Kevin McCarty of Sacramento, and former state Sen. Dean Florez.
The Democratic Senatorial Campaign Committee announced on Sunday that it was backing Feinstein, not surprising since the group supports incumbents. She received several high-profile endorsements when she kicked off her reelection bid last week.
Political strategists say that although she enjoys institutional advantages, she can't take for granted that this may be her toughest race since 1994, at a time when segments of the Democratic electorate are agitating for change and grossly dissatisfied with the status quo. Making her case to younger voters who are unfamiliar with her record as well as the most liberal wing of the party that is hungry for a flame-throwing critic of Trump will be key.
"It will be Dianne Feinstein's job to reassure voters who are concerned … that she is very much in tune with current issues and the current concerns of California," said Darry Sragow, a veteran Democratic consultant who ran her unsuccessful 1990 campaign for governor. He said this was particularly true for younger voters. "The oldest of voters under 30, they would have been five when she was first elected [to the Senate]. They were watching 'Sesame Street,' not 'Meet the Press.' She has her work cut out for her."
Among De León's greatest challenges is likely to be fundraising. As a state party leader, he has cultivated relationships with some of the most prominent donors in the state, but some might be wary of challenging a sitting senator. De León lacks a statewide donor base. And the roughly $3 million he has parked in state accounts can't legally be transferred to a federal race.
Other competitors may get in the ring, notably billionaire environmental activist Tom Steyer, who said Sunday he is considering a run. Feinstein is also being challenged by attorney Pat Harris, who announced a long-shot bid in August on a platform that includes support for single-payer healthcare and a pledge that he will only take campaign contributions from individual donors.
De León's greatest strength could be his life story, which may appeal to voters hungry for change, noted Dan Schnur, a political communications professor at USC.
"It would be an uphill fight for any challenger, but if anybody could pull it off, it would be someone with a personal and political biography like Kevin de León," he said.
The child of an immigrant single mother, De León, 50, spent much of his childhood trekking from his humble home in the Logan Heights neighborhood of San Diego to the city's wealthier enclaves, where his mother worked as a house cleaner.
His upbringing would prove influential in shaping the political career that was to come.
He worked on campaigns and for labor unions, and won a state Assembly seat in 2006. In 2010, he moved to the Senate and was elected leader of that chamber in 2014 — the first Latino to hold that position in more than a century.
In the state Capitol, he has embraced high-profile legislative lifts, pushing state-sponsored retirement plans for low-income workers and background checks for ammunition purchases.
He has been a central figure in California's efforts to combat climate change, including the setting of aggressive targets to reduce greenhouse gas emissions and extending the state's landmark cap-and-trade program.
But his environmental advocacy has also led to high-profile setbacks. A 2015 proposal to slash petroleum use in the state by 50% by 2030 collapsed, and this year, his bill to phase out by 2045 all fossil-fuel use for generating energy sputtered in the legislative session's final days.
He also has eagerly embraced positioning California as the heart of the "resistance" against Trump and the federal government.
That posture was most evident in his signature legislation of the year, the "sanctuary state" measure, which limits state and local law enforcement's cooperation with federal immigration officials.
12:50 p.m.: This article was updated with Steyer saying he might run.
11:35 a.m.: This article was updated with information about endorsements of De León.
10:55 a.m.: This article was updated with quotes from De León and Bill Carrick.
10:15 a.m.: This article was updated with details of De León's announcement.
12:07 a.m., Oct. 15: This article was updated with details of De León's clean energy measure this year.