LOS ANGELES — The mailers and online ads vividly paint David Kim as a right-wing extremist, accusing him of running for a House seat in California “with QAnon-MAGA support” from “QAnon Republicans.”

But Mr. Kim is not a Republican. He’s a progressive Democrat who supports “Medicare for all” and a Green New Deal. And the attacks come from a fellow progressive Democrat, Representative Jimmy Gomez, who is fighting to keep his seat in Congress.

The vitriol in what is normally a quiet race for a decidedly safe Democratic seat illustrates how liberal California, of all places, has become home to some of this year’s most vicious political mudslinging — and not across party lines.

Unlike a vast majority of the country, where voters are mulling the yawning ideological gaps between Republicans and Democrats on their midterm ballots, California has a top-two open primary system, which means two Democrats can — and often do — square off against each other in general elections. And in many cases, those candidates prove strikingly similar on policy, forcing them to dig deep to distinguish themselves.

Lately, it’s grown pretty nasty.

Democrats are running against Democrats in six House races, 18 state races, and dozens of municipal and local elections around California in November. In many contests, the candidates have resorted to extreme and divisive language, in a reflection of the growing polarization of American politics.

That’s particularly the case in azure-blue Los Angeles, where nearly every elected office is held by a Democrat, only a single Republican has served as mayor in the past half-century, and an explosive racism scandal involving three members of the City Council plunged the city’s political world into chaos just weeks before the election.

Take the race to be the city’s next controller, typically a dull contest with few, if any, pyrotechnics. But this year, Paul Koretz, a progressive city councilman with more than three decades in public office, has taken to calling his opponent, Kenneth Mejia, “dangerous,” saying he is an antisemite and an “anarchist” who is little different from the rioters who stormed the Capitol on Jan. 6, 2021.

“He’s the phoniest Democrat you can find,” Mr. Koretz said in an interview of Mr. Mejia, a relative political newcomer and former Green Party member who has supported a reduction in police funding and backs a national tenants’ bill of rights.

Mr. Mejia’s campaign manager, Jane Nguyen, called Mr. Koretz’s accusations “ridiculous smears” and said he was “out of touch.” Since the leak this month of a recording of City Council members in a discussion that involved offensive comments, she and other allies of Mr. Mejia have sought to portray Mr. Koretz as a racist, accusations he denies.

Some Democrats worry that the poisonous environment is bad for party unity.

“There are wild charges going back and forth about whether one candidate is a closet conservative and one is a closet Marxist,” said Garry South, a longtime Democratic strategist. “I just don’t think these runoffs between Democrats should turn into a derby about who can accuse the other of being the most extreme. That’s not a healthy debate to have.”

Mr. South and other consultants pointed out several other races where the animosity was at full pitch, including a contest for a City Council seat on the wealthy Westside of Los Angeles. One candidate, a centrist Democrat with a background in employment law, has tried to link her progressive opponent, a criminal defense lawyer, to pedophiles and rapists. That lawyer, in turn, has called her a racist.

In a race for a State Senate district encompassing parts of Downtown and South Los Angeles, two Democrats with nearly identical policy positions have accused each other of being pro-business and anti-tenant.

“It’s a race to the bottom,” Mr. South said.

For more established candidates, the negative messaging may come from a place of desperation: Even before the controversy consumed City Hall early in October, an anti-incumbent mood seemed to have settled over Los Angeles.

In the June 7 primary, two sitting City Council members lost decisively to inexperienced challengers, one by a wide enough margin to preclude a runoff.

In their primary, Mr. Mejia walloped Mr. Koretz by nearly 20 percentage points, though he fell short of the 50 percent threshold that would preclude a runoff. Afterward, Mr. Mejia gained several key endorsements, including from The Los Angeles Times and Councilman Mike Bonin, whose son was the subject of some of the racist comments at the heart of the unwinding scandal.

Mr. Koretz, by contrast, had been endorsed by all three City Council members caught on the now-infamous recording. After the scandal broke, his campaign website was scrubbed of mentions of their support, cached versions of the site show.

In their race for Congress, Mr. Gomez and Mr. Kim have a history.

In 2020, Mr. Gomez, who has served in the House since 2017, also faced Mr. Kim, an immigration lawyer. That time, Mr. Gomez won the primary by almost 30 points and went on to the general election without paying much, if any, attention to his rival. “He never even mentioned my name,” Mr. Kim said over a breakfast burrito at a restaurant this month.

But Mr. Gomez ended up winning the 2020 general election by a surprisingly slim margin of six points. In April 2021, he asked the Democratic Congressional Campaign Committee to inject money into the contest for what has traditionally been a safe seat for the party, telling donors he was “in a very tough race,” HuffPost reported.

This past August, Mr. Gomez’s campaign unveiled a website titled “Who Is the Real David Kim?” that accuses him of failing to support democracy and of hiding a “QAnon MAGA endorsement”; a core falsehood of QAnon is that a cabal of Satan-worshiping pedophiles is trying to control politics and the news media. In October, Mr. Gomez has sent out three mailers juxtaposing photos of Mr. Kim with an image of Jan. 6 rioters climbing up the walls of the Capitol, and has paid for internet ads with side-by-side pictures of his rival and former President Donald J. Trump.

The QAnon reference, explained Steven Barkan, a consultant for Mr. Gomez’s campaign, stems from the fact that, in 2020, Mr. Kim asked for and received endorsements from the losing candidates in the primary. One of them, Joanne Wright, a Republican, turned out to have embraced conspiracy theories, and had a “Q” image on her Twitter page before being kicked off the platform. Mr. Barkan said that Ms. Wright’s views were exposed before the primary, and he argued that Mr. Kim either knew or should have known whom he was dealing with. (Ms. Wright did not respond to requests for comment.)

“It is correct to say we don’t think he’s QAnon,” Mr. Barkan conceded. “But he ran with QAnon support. It is serious when people like Kim give credibility to QAnon.”

Some of the Gomez campaign’s attacks have also centered on the fact that Mr. Kim was once a registered Republican; Mr. Kim says he was raised in a Republican family but long ago changed his registration.

Mr. Kim called the advertising campaign a dirty tactic and said the endorsements, which included one from a Democrat he defeated, showed that he was “a unifying candidate and leader” willing to work with people who hold different viewpoints. He also pointed out that the endorsement occurred in the 2020 race, yet was being misleadingly presented as if it happened in the current campaign.

“People like Jimmy continue to add to the extreme polarization of our country and government,” Mr. Kim said.

Bill Przylucki, the executive director of Ground Game LA, a nonprofit group that promotes progressive politics and candidates, frowns on messaging that associates left-wing candidates with extremists at a time when more than 370 Republican candidates for influential offices nationwide have cast doubt on the legitimacy of the 2020 presidential election.

“Given the national picture, I think that’s a bad strategy overall for the Democrats,” Mr. Przylucki said, comparing the approach to “red-baiting.”

Some political observers note that it has scarcely been a decade since California moved to an open primary system, and say that some adjustment is necessary. But Mr. Przylucki argued that the hostilities emerging in the current system called for a complete rethinking of traditional Democrat-versus-Republican politics.

“Democrats are trying to figure out what it means to be a party in a place like Los Angeles,” he said. “Or whether it even makes sense.”