Sorry for this trip down memory lane, but I thought it illuminating as I Googled around both because Hayden's a helluva writer with a helluva past, and he sets up some understanding of the current movement in California.
De Leon was a Hillary supporter - but he also symbolizes some of the multicultural movements and yearnings. Feinstein is largely an old-schooler, and certainly not pleasing to the typical liberal Californian on her security stands (though CA's huge defense industry undoubtedly has her back).

This isn't quite New York's version of Ocasio - it's a much more careful shift, even if the vast majority came out for Kevin de Leon in the endorsement (and as Hayden noted, Bernie had few endorsements, which made his movement hard to seriously consider once in office).

The fracking biz also indicates a fault line - the "uncivil" left will also pick an unwinnable nonsensical point-of-view to push, and punish those in disagreement unmercifully. Fracking was one of them, the push to full Medicare-for-all another. But De Leon undoubtedly isn't falling into that minefield. Despite some blunders, his positions seem to be (in my quick scanning of positions and background) firmly rooted in opportunity, and it's hard to see a downside to that.

My first reaction was to go with the tough old broad who'd fought Dem battles for so many years, but Feinstein's not a Pelosi. De Leon's endorsement of Hillary helps me transition -  he's not a "burn the boats" guy. (And I'm fine with Ocasio's mini-revolution in the Bronx - the House has a different function than the Senate, and is more a long-term, many-voices pressure that I welcome).

So while I'm sure Feinstein won't take this sitting down, I'm guessing this endorsement is a positive step for California to figure out its next generation leaders and position itself for both the demographic/economic changes as well as the stark political realities in both Sacramento and Washington.