Running as a progressive champion may be all Kevin de León can do. Unlike Newsom and Harris people don't know who he is and he doesn't have a record like Harris. SurveyUSA had Feinstein ahead 52%-7% among Democrats and 22%-5% among independents. He isn’t competing for the 35-42% of the voters who lean right. An incumbent like Feinstein has a huge advantage due to her long record in the job and her popularity. There are also a percentage of Democrats who might be inclined to vote for the more progressive candidate but would support an incumbent over a challenger. It'd be disloyal to Feinstein to vote against her.
de León is trying to get enough of the voters who lean left and hoping for a fractured Republican field. It’s possible he gets that. Loretta Sanchez did, although she did so with no incumbent, high Democratic turnout, low Republican turnout, and running to Harris’ right, not her left. The general election is impossible. Republicans will vote heavily Feinstein, probably better than 80%-20%. One flank of de León's campaign is how awful Republicans are. He’s actively telling them he doesn’t want their votes and vote Feinstein. While some will leave the ballot blank, most will vote for one of the two candidates.California had 13.7% less votes in the 2016 senate race than the presidential race. Most other races were in 2% range. The two right leaning Presidential candidates got 35% of the California vote. So, at most, 1/3 of right leaning voters skipped the race. He’d have to win Democrats and independents by 25% to overcome that. And right now he’s losing Democrats by more than 7 to 1.