So why are they positioning de León here? He has nowhere else to go. His resume is as a progressive champion. His reason for running is to be a progressive champion. Dianne Feinstein is vulnerable from a progressive position.
Don't they realize de León can't win if all he's running is as a progressive anti-Trump candidate? I'm sure they do but it's possible that progressives think that over half of California's voters will support a progressive.
Why run if they don't? I can give a few possibilities. Maybe the objective isn't for de León win but highlight progressive policies and move Feinstein and the rest of California to the left. De León is termed out of the state senate so why not do that?
What else? It may be macabre to say this but Dianne Feinstein will be 85 next year. While I haven't heard she isn't healthy that's years past the average life expectancy. It's possible that she withdraws due to illness or death before the primary deadline in March. While beating Feinstein might be nearly impossible de León could win a race she's not in. If that happens after the filing deadline Feinstein will still be on the ballot. You can't get off the California ballot once you're on. Yes, even if you die. If she were to finish top two, she'd advance to the November election, even though she wouldn't be able to serve. Either of those scenarios could put de León in the U.S. Senate.
If Feinstein is re-elected but can't serve her entire term de León will have an excellent argument that he should be appointed to the seat, since he will have gotten a lot of votes. And he'd have an excellent argument in a special election, along with name ID and a volunteer/donor base.
I'm not wishing ill on Dianne Feinstein. May she live a long life. But if I've considered the possibility de León's people have also.