The Democrats lost in California for various reasons, including issues specific to the state and challenges posed by campaigning during the pandemic. However, a significant factor was the strength of Republican criticisms, many of which were untrue or exaggerated. They claimed that Democrats supported socialism, wanted to defund the police, and aimed to eliminate private health insurance.

These attacks, largely led by Mr. Trump as part of his plan to get re-elected, happened during a period when some areas in California were experiencing protests against police misconduct. Some of these protests turned into situations involving breaking glass and clashes with the police, getting extensive coverage on local TV.

"Republicans accused Democrats of being socialist or communist and wanting to defund the police," said Harley Rouda, a Democrat from Orange County who lost to Republican Michelle Steel, a member of the Orange County board of supervisors. "In my view, our party did not do a good enough job countering that narrative. We secured victory in 2018 and regained control of the House because of people like me—moderates—who flipped traditionally Republican seats."

Republicans argued that Democrats' efforts to present themselves as moderates were undermined by the party's leftward shift and the impact of the protests.

"It was very simple for us to highlight the differences," stated Jessica Millan Patterson, the head of the California Republican Party. She mentioned, "The protests were occurring everywhere, resembling a war zone."

Despite all this, election results are influenced by numerous factors, especially in a campaign that unfolded amid a deadly pandemic and with a polarizing figure like Mr. Trump dominating the political discourse.

Democrats also pointed to a national party policy, established during the pandemic, to avoid door-to-door canvassing, which they believe negatively impacted their campaign. Presumably, this won't be a factor in 2022.

"The primary issue in our campaign was the lack of canvassing," said Representative T.J. Cox, a Democrat representing the San Joaquin Valley who lost to Republican David Valadao, the candidate he had unseated in 2018. He likened it to playing on a football team that had been instructed not to pass.

Christy Smith, a Democrat representing northern Los Angeles County, explained that she faced a setback by following public health guidelines. The district, previously won by Democrat Katie Hill in 2018, became open after Hill resigned amid allegations of an inappropriate relationship with a staff member.

“We couldn't participate in a forum or a town hall,” expressed Ms. Smith. “That’s my preferred way to campaign.”

California's inconsistent efforts to combat Covid-19 had a negative impact in Republican-leaning districts where there was public resistance to mask mandates and discontent toward the state's Democratic governor, Gavin Newsom.

“Covid is a concern for everyone,” remarked Sam Oh, a Republican consultant for winners Young Kim and Ms. Steel. “But we are working to find a way to support small-business owners in maintaining their livelihoods. This is crucial, and Democrats seem insensitive to this.”

Republicans, analysts noted, successfully recruited formidable candidates, a crucial factor in any election. Among them were Ms. Steel and Ms. Kim, who will be among the first Korean-American members of Congress, and Mike Garcia, a former military pilot who won a special election in May to replace Ms. Hill and then defeated Ms. Smith in November.

In 2018, Democrats targeted seven Republican-held seats and won them all, significantly reducing the size of California’s Republican congressional delegation. This success suggested that Democrats were making inroads in once-Republican areas, providing a strategy for competing in swing districts nationwide.

However, Republicans achieved success this time by emphasizing themes that resonate with moderate voters, especially in places like Orange County: high taxes, government intrusion, and law and order. Democrats acknowledged that national debates, particularly on issues like socialism, crime, and messaging, hurt them, especially among Latino and Asian-American voters.

Dan Sena, the first Hispanic executive director of the Democratic Congressional Campaign Committee, admitted, “I think we undervalued the strength of the attack. The national socialism messaging, combined with the crime messaging, was a death by a thousand cuts in a place like California.”

Ms. Smith expressed frustration at campaigning in an environment where Republicans persistently spread false narratives, and Democrats struggled to address this challenge. “We never got our hands around it,” she said.

For California Republicans, these victories offered a rare positive note for a party experiencing decline in the state. Sam Oh stated, “We now have a blueprint that shows that these really dynamic candidates can win with a presidential turnout, running in a polarized environment. We are in an incredibly good position looking forward.”

In a potential indication of a shift, Mr. Newsom is confronting a recall campaign, largely due to his handling of the pandemic. While it's unlikely he will be removed from office and replaced by a Republican, it's not entirely impossible, as demonstrated when Arnold Schwarzenegger, a Republican, became governor in 2003.

“California is at a turning point,” remarked Ms. Patterson. “People are waking up to what Democrats are doing here. This was a referendum on the actions of California Democrats and the governor's impact on the state.”

Republicans contended that the results, both in California and nationwide, strongly suggested that many voters were rejecting the policies advocated by the left-wing Democrats.

“Democrats said they were going to pick up 10 to 15 seats in the House,” stated Torunn Sinclair, a press secretary for the National Republican Congressional Committee. “Obviously, they are missing something. It has to go back to these far-left policies that people just didn’t want—‘Medicare for all,’ defunding the police.”

The margins of victory in California's pivotal districts were narrow for the Democratic winners in 2018 and for the Republican winners in 2020.

“These districts — including mine — were exceptionally challenging to flip,” stated Representative Katie Porter, one of the Orange County Democrats who succeeded in 2018 and was re-elected in 2020. “These are traditionally Republican districts, and they were always anticipated to be fiercely contested races.”

Democratic strategist Darry Sragow mentioned that the state Republican Party didn't lose ground in 2020. “But it’s very difficult to argue they are better off,” he added. “Those congressional districts were always Republican until 2018 when the Democrats picked them off.”

The four Democrats who lost are considering seeking rematches in 2022, although district lines will be redrawn as part of the decennial redistricting process before the next election. Both sides acknowledged that the races would be closely contested once again.

“It was a pause,” said Mr. Cox, the San Joaquin Valley Democrat. “But keep in mind we lost by less than 1 percent, without running a field campaign. So it’s not surprising that with these factors at play that we came up a little short.”