BFFs in D.C.? Apparently not U.S. Sens. Dianne Feinstein and Kamala Harris of California.

According to reports, Feinstein, the senior senator from the Golden State, will throw a fundraiser next month for Democratic 2020 presidential frontrunner Joe Biden -- and not for Harris, another White House contender whose poll numbers suggest she could probably use some high-profile help.

Feinstein and husband Richard Blum will co-host the Biden event Oct. 3 in San Francisco, according to a copy of the invitation obtained by CNBC. Feinstein first gave her backing to Biden in January, months before the former vice president made his campaign official in April.

FEINSTEIN BACKING BIDEN FOR PRESIDENT, SNUBBING FELLOW CALIFORNIAN KAMALA HARRIS: REPORT

“My candidate would be Joe Biden,” Feinstein told CNBC back then. “I watched him as vice president. I’ve seen him operate. I’ve seen him perform and I think he brings a level of experience and seniority, which I think is really important.”

The early evening reception will be the first that Feinstein hosts for Biden in the 2020 election, CNBC reported. Her husband has already participated in several fundraising events for Biden, who campaigned for Feinstein in 2018, when she was re-elected by a landslide.

Feinstein has repeatedly faced questions about why she prefers Biden -- who was her Senate colleague for nearly two decades -- over Harris, her home state’s junior senator.

Harris and Biden famously butted heads during Democratic presidential debate earlier this year when Harris challenged Biden’s stance decades ago on the issue of desegregation school busing when he was a U.S. senator from Delaware.

“She’s brand-new here,” Feinstein told the Los Angeles Times about Harris in January when asked if she would support a Harris run for the White House. “It takes a little bit of time to get to know somebody.”

She also told the Washington Examiner in May when Biden was chairman of the Senate Judiciary Committee he made her the “first woman” to serve on the panel.

“I love Kamala, I appreciate her,” Feinstein said, “but that’s what I’m going to do. I feel very loyal to him."

Ticket prices begin at $1,000 and cap out at $2,800 per person for the October event. Several other hosts will join Feinstein and Blum on the stage, including Denise Bauer, a former ambassador to Belgium, the emailed invitation said. Feinstein and Blum are not listed as hosts for a fundraising lunch to be held in nearby Palo Alto, Calif., earlier that day.

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Harris, who previously served as California's attorney general -- and was district attorney of San Francisco, where Feinstein was once the mayor -- was elected to the Senate in 2016. In 2004, Feinstein said she would not have endorsed Harris for San Francisco district attorney if she had known Harris wouldn’t seek the death penalty for a gang member who killed a police officer, the Washington Examiner reported.

Fox News’ Brie Stimson contributed to this report.