During her career, she was re-elected five times, winning in 1994, 2000, 2006, 2012, and most recently in 2018, where she defeated fellow Democrat Kevin de León with 54.2% of the vote.

Media sources speculated for years that Dianne Feinstein might choose not to seek reelection in 2024 or resign before the end of her term, owing to her age, reports that her cognitive state was declining, and her decision not to take the position of Senate president pro tempore in the 118th Congress, third in line for the presidency, even though she would customarily have been offered the role as the most senior member of the majority caucus.

[45] In January 2023, with the question of Feinstein's reelection decision still open, U.S. Representative Katie Porter announced that she would run for the Senate.

She later gained national fame for her progressive politics, and frequently went viral online for her pointed questioning of corporate executives in congressional hearings, often while using a whiteboard.

[47] Two weeks later, Porter was joined by another Democratic member of the House, Adam Schiff, who said that he had consulted with Feinstein before entering the race.

[49] Schiff has not faced a competitive election since 2000, as his Los Angeles-based district became significantly more Democratic during the 2000 redistricting cycle and has been considered a safe seat ever since.

[59] All three have faced controversies that could damage their campaigns: Porter has been accused of mistreating congressional staff, Lee's age was seen as a potential issue, and Schiff was expected to face opposition from progressives due to his past support for overseas military intervention and for taking donations from groups affiliated with the oil, payday loan, and pharmaceutical industries, though he has declared he would not accept funds from corporate PACs in his Senate campaign.

[60][59] Schiff has also been criticized for listing his primary residence as Montgomery County, Maryland, in tax documents, though his campaign maintains that he lives in Burbank, California.

[66] This scenario seemed to come to pass when former Google executive Lexi Reese joined the race in June 2023; her aides told Politico she would spend a "significant" amount of her own money on her campaign.

[67] However, Reese made little impact on the race and dropped out months later; though she raised $2 million, much of it self-funded, she wrote that this was "just not enough to run a state-wide campaign.

[43][40] Feinstein faced calls to resign throughout 2023 due to reports of her declining health, including from U.S. Representative Ro Khanna.

[69] Possible appointees speculated by media sources included Barbara Lee, Los Angeles mayor Karen Bass, San Francisco mayor London Breed, Los Angeles County supervisor Holly Mitchell, Secretary of State Shirley Weber, and talk show host Oprah Winfrey, though Bass, Mitchell, and Winfrey said they would not be interested.

[74] Newsom was expected to quickly appoint a successor, as a crisis over a potential government shutdown necessitated a united Democratic front in the Senate.

[75] In addition to those already mentioned, possible successors speculated by media sources included PolicyLink founder Angela Glover Blackwell, former state assemblywoman Autumn Burke, EMILYs List director Laphonza Butler, State Controller Malia Cohen, California Supreme Court justice Leondra Kruger, Bay Area Rapid Transit Board of Directors president Lateefah Simon, and U.S. Representative Maxine Waters.

Butler responded that she had lived in California for many years before moving to the D.C. metropolitan area in 2021, and that she still owned a home in Los Angeles; promising to re-register to vote in the state.

Two other Republican candidates, healthcare executive James Bradley and attorney Eric Early, both of whom had previously made several unsuccessful bids for office, also attracted some media attention and support in polls.

Lee received the most delegate votes, narrowly outpacing Schiff; however, neither candidate came close to reaching the 60% threshold necessary to win the endorsement.

[93] In the months after his announcement, Garvey steadily rose in polling, heightening Republicans' odds of getting a candidate through to the general election.

[94] Garvey largely avoided taking positions on political issues, spent no money on television ads, and did little in-person campaigning.

[98] Politico, FOX 11 Los Angeles, and USC Dornsife held a debate on January 22, 2024, which included Schiff, Porter, Lee, and Garvey.

[102][103] KNBC and KVEA aired a debate in conjunction with Loyola Marymount University on February 20, which was broadcast on NBCUniversal-owned stations statewide and on KCRA, Sacramento's NBC affiliate.

[104] Topics included the minimum wage, the federal budget deficit, military spending, the Mexico–United States border crisis, climate change, extending the life of the Diablo Canyon Power Plant, and the proposed regulation of artificial intelligence.

[268][269] After her loss, Porter remarked that the election was "rigged by billionaires," referring to a $10 million independent ad campaign attacking her that was funded by cryptocurrency supporters.

[271] Porter's use of the word "rigged" was criticized, with some comparing her statement to Donald Trump's false claims of fraud in the 2020 election.