She says it's to avoid an intraparty fight, but her effort if successful will thwart her longshot rival's very real chance at capturing the party nod when the committee votes Saturday.
A 26-year incumbent like Feinstein wouldn't ordinarily find herself in this position but does for a couple of reasons: The party has moved farther left in the Trump era, and Feinstein hasn't spent time courting the activists who make up the executive board, many of whom have long been skeptical of her. De Leon, meanwhile, has engaged with them for years as a state senator from Los Angeles and former leader of the chamber.
A party endorsement would be a desperately needed boost to his campaign and an embarrassment to Feinstein.
She made the case against endorsement to party leaders through letters and emails featuring political allies including Planned Parenthood of California chief executive Crystal Strait, labor icon Dolores Huerta and former state party Chairman John Burton.
Six Democratic candidates seeking to flip Republican-held U.S. House seats critical to the party's hopes of taking back Congress also implored delegates not to weigh in. Feinstein and fellow California U.S. Sen. Kamala Harris hosted a fundraiser Wednesday night in Washington for those six candidates and two others.
"A divisive party endorsement for U.S. Senate would hurt all down ballot candidates and our ability to turn out Democrats we desperately need to vote in November," their letter warned. "We urge you to vote no endorsement and let the respective campaigns make their own case to voters for the general election."
The letter angered de Leon backers.
"They're trying to send some sort of message that if you support Kevin de Leon you'll be seen as a spoiler and not playing nice in Democratic Party politics," said RL Miller, chair of the party's environmental caucus and an executive committee member.
Members like Miller say de Leon has more progressive bonafides as the author of California's so-called sanctuary state law and bills to expand clean energy. Feinstein, for her part, voted against ending a government shutdown earlier this year without protections for young immigrants living in the country illegally after some protested outside her office. She also recently reversed her long-held support for the death penalty, an issue she used in the 1990s to show her independence from her party.
De Leon nearly won the endorsement at the party convention last winter but fell just short of the 60-percent threshold needed in a vote by thousands of delegates. Feinstein later demolished him in the June primary, 44 percent to 12 percent. Under California's top-two system, he still advanced to the November election because he finished second in the field of more than 30 candidates.
Round two of the party endorsement fight comes this weekend, when about 360 executive board members gather in Oakland. De Leon's campaign and party officials believe he's in reach of capturing the endorsement. Beyond garnering headlines, it means the party would spend money on de Leon's behalf and promote him alongside other Democratic candidates such as gubernatorial nominee Gavin Newsom.
Winning the endorsement is all but essential to de Leon's ability to broaden his name recognition and run a credible campaign against Feinstein, one of California's best-known politicians who has millions more in the bank to make her case.
As the top Democrat on the Senate Judiciary Committee, Feinstein will be center stage in the battle over President Donald Trump's Supreme Court nominee.
Feinstein's campaign disputes that she's worried about de Leon securing the endorsement and instead, said she's adhering to party Chairman Eric Bauman's call for candidates in Democrat-on-Democrat races not to divide the party by seeking the endorsement. The two Democrats running against each other for lieutenant governor agreed not to seek an endorsement.
Mike Levin, who is running to replace retiring Republican U.S. Rep. Darrell Issa, is among the Democrats who signed Feinstein's letter. He said he did so to promote party unity and ensure Democrats are focused on the right races.
"The Party's focus should be on flipping as many red to blue seats as possible, rather than on races where we are guaranteed of having a Democrat win in November," he said in an email.
De Leon, meanwhile, is working hard to make his pitch to delegates. Spokesman Jonathan Underland said California Democrats deserve a debate on issues that they haven't had during Feinstein's more than quarter-century in office.
He called any criticism of de Leon's choice to seek the endorsement "absolutely comical," noting that he earned the support of 54 percent of delegates in February and has more support among local Democratic clubs.
For Bauman, the party chair, the more energy spent on a Feinstein-de Leon battle, the less left for more important Democratic fights. "A key part of what I am trying to do is minimize the intra-party tension and battling that's going on," he said.
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