Trump: Dems playing destructive 'con game' with Kavanaugh
Sanders won't endorse Feinstein
Though Sanders has put himself in the middle of other primaries in this cycle, he's steering clear of California, where Feinstein is being challenged from the left.
"It's an issue for the people of California," Sanders told The Hill.
Asked if that meant he would stay out of the race, Sanders responded, "Yeah."
While Sanders says the California race is for the people of that state to decide, on Thursday he announced that he would back liberal candidate Marie Newman in her primary challenge against Rep. Daniel Lipinski (Ill.), a seven-term Democratic incumbent.
Newman has also won endorsements from liberal Reps. Jan Schakowsky (Ill.) and Luis Gutiérrez (D-Ill.), though House Democratic leaders are backing Lipinski.
A spokesman for Feinstein did not respond to a request for comment on Sanders's remarks.
It's not a huge surprise that Sanders would not back Feinstein in the primary, and Feinstein may be relieved that the popular liberal is not endorsing her rival, Kevin de León, president pro tempore of the California state Senate.
Feinstein failed to win an endorsement at California's state Democratic convention last month, winning just 37 percent of the vote. De León won 54 percent, which also fell short of the 60 percent necessary to win the endorsement.
The challenger was helped by the efforts of Sanders supporters who have made a concerted effort to exert more control over local parties.
De León is advocating for some of the same proposals that ignited Sanders's presidential campaign two years ago such as universal Medicare and free college tuition. He argues that Feinstein is too much of a centrist, particularly in the era of President Trump.
"California Democrats are hungry for new leadership that will fight for California values from the front lines, not equivocate on the sidelines," he said in a statement after the state party convention.
Many Sanders backers were left disillusioned after the 2016 Democratic presidential primary, during which they believed that party officials such as former Democratic National Committee Chairwoman Debbie Wasserman Schultz, who represents Florida in the House, tipped the scaled in favor of Hillary Clinton. Clinton built up an insurmountable delegate lead because so-called superdelegates, who were comprised of Democratic lawmakers and party officials, backed Clinton overwhelmingly over Sanders.
Feinstein, who is running for her sixth term, was one of Clinton's staunchest backers, and she put pressure on Sanders to drop out of the race.
"He ought to be able to read the signposts as well as anybody else, and if he did that, he would know that it's all over," Feinstein said in May of 2016.
Sanders, however, didn't concede the race until July.
Feinstein, throughout her career, has been viewed as more centrist than many of her Democratic colleagues.
She has been a strong proponent of giving intelligence agencies wide-ranging authority to conduct surveillance against suspected terrorists.
She voted, for example, to reauthorize the warrantless spying program as part of the Foreign Intelligence Surveillance Act and to confirm former Republican Sen. Dan Coats (Ind.) as the director of national intelligence.
Sanders voted "no" on both questions.
An analysis by FiveThirtyEight, a publication that specializes in political statistics, found in October that Feinstein had voted in support of Trump's agenda 31 percent of the time, a higher percentage than a lawmaker from such a liberal state would be expected to.
Her record has caused friction with more liberal Democrats.
California Rep. Ro Khanna (D) called on her to "move on" last year and criticized her for not being a stronger advocate for privacy or backing universal Medicare.