Mr. Fitzgerald ran for Congress in California as a Democrat in 2010 and 2012, but did not receive the party’s endorsement in either case. He is running as a Republican in November but said he identifies more as an independent.

“Am I a Republican?” Mr. Fitzgerald said. “What is a Republican anymore?”

Mr. DeSaulnier said in an interview on Thursday that he believed California voters — more than 36,000 of whom voted for Mr. Fitzgerald — were unaware of Mr. Fitzgerald’s views when they went to the polls. “People see ‘R’ and they see ‘D’ and that’s how they vote,” he said.

Mr. DeSaulnier, who was elected to office in 2014 and received 68 percent of the vote in the June primary, said anti-Semitism is an “ugly and evil” set of beliefs that he does not believe the residents in his district will ultimately support.

On a podcast called “The Realist Report,” released last month and hosted by John Friend — a self-described independent journalist whom the Anti-Defamation League has called an anti-Semite — Mr. Fitzgerald lamented “Jewish control and supremacy.” In a blog post on his website, Mr. Fitzgerald wrote that Jewish people are behind a push for “multiculturalism, diversity and inclusiveness” throughout the United States and other “once predominantly white nations.”

Media Matters, a liberal nonprofit publication, drew attention this week to Mr. Fitzgerald’s podcast appearances.

Mr. Fleming, the state Republican Party spokesman, said that if only one Republican joins the race in California’s open primary system — often called the “jungle primary” — the state party’s bylaws say the organization will automatically endorse that candidate. He said that before the California Republican Party endorsed Mr. Fitzgerald in late March, one of the few “red flags” on his campaign website was conspiracy theories about the Sept. 11 attacks. Mr. Fleming said he would have alerted state Republican leaders if he had seen those statements before the endorsement process.

Ron Nehring, a former chairman of the California Republican Party and spokesman for Senator Ted Cruz’s 2016 presidential campaign, said he was the first person to alert the organization to the views expressed on Mr. Fitzgerald’s campaign website, prompting discussion about disavowing the candidate. Mr. Nehring said he planned to propose a change to the state Republican Party rules to eliminate automatic endorsements.