Schiff is making progress in the battle for Feinstein’s seat in the California Senate.

Rep. Adam Schiff, the lead prosecutor in Donald J. Trump’s first impeachment trial, advanced to the general election Tuesday night, according to The Associated Press.

Election results showed Steve Garvey, a Republican political novice and former Major League Baseball star, competing with Rep. Katie Porter, an Orange County Democrat, for the second spot in the November runoff.

Representative Barbara Lee, a Democrat and noted Bay Area progressive, was in a distant fourth place.

November’s general election will determine who will replace Dianne Feinstein, who held the seat for more than three decades until her death in September.

During the campaign, Ms. Porter, 50, had accused Mr. Schiff, 63, and his supporters of manipulating Mr. Garvey’s turnout to facilitate an election in November. Mr. Schiff, who had raised more than $30 million last month, spent lavishly on statewide ads comparing Mr. Garvey to Mr. Trump and portraying him as Mr. Schiff’s main opponent.

The strategy encouraged the state’s Republican voters to rally behind the 75-year-old Garvey, who is still recognized from his days with the Los Angeles Dodgers and San Diego Padres more than three decades ago. And it elevated Mr. Schiff to the leading Democrat for an electorate dominated by liberals.

If Mr. Garvey secures second place in the fall general election, the electoral calculations will be overwhelmingly in Mr. Schiff’s favor. Democrats have a 2-to-1 voter advantage in California, and the state hasn’t elected a Republican to statewide office since Arnold Schwarzenegger was re-elected as governor in 2006.

If Ms. Porter takes second place, California will face an intraparty clash between liberal and moderate Democrats, as well as a generational divide given Ms. Porter’s popularity among younger voters.

Turnout for Tuesday’s primary was noticeably low, putting some resulting decisions in the hands of an electorate that is older, whiter and generally more conservative than the state’s entire voting population. Republicans, who make up less than a quarter of California’s registered voters, had cast more than 31 percent of the votes when the final day of the primaries began.

When the race for the Senate began more than a year ago, the conventional wisdom was that the general election would come down to Mr. Schiff and Ms. Porter, two well-known Democrats with formidable records in Congress and fundraising prowess.

Mr. Schiff, a former federal prosecutor and 12-term Congressman from Burbank, rose to national prominence as ranking member and then chairman of the House Intelligence Committee. As chairman, he led the first impeachment inquiry into Trump’s efforts to solicit election aid from Ukraine.

Ms. Porter, a former law professor who has represented southern Orange County since 2019, gained wide recognition for her whiteboard tutorials on national issues and her blunt statements about powerful leaders and business leaders during congressional hearings.

A prodigious fundraiser, Mr. Schiff had collected early endorsements from most of his Democratic colleagues in California’s large House delegation, many of whom he had previously endorsed. Among them was former Speaker Nancy Pelosi, for whom Mr. Schiff had long been a trusted lieutenant. Ms. Porter leveraged grassroots fundraising and her popularity with younger progressives, many of whom viewed Mr. Schiff as too big of an establishment player.

Then, in October, Mr. Garvey entered the race and immediately became the best-known Republican in a field of about two dozen candidates. Mr. Garvey still has significant name recognition among older Californians, and Republican officials welcomed him as a name that could mobilize their voters.

In three televised debates, Mr. Garvey presented himself as a unifying presence and declined to say whether he would vote for Mr. Trump or accept his support. His policy positions were vague and he skipped most other candidate forums.

But Mr. Schiff’s ads boosted his standing in the polls, as did a $10 million ad campaign against Ms. Porter launched in recent days by tech billionaires and cryptocurrency interests.