When polling you want to heavily poll the "4s" and "5s," especially in a mid-term. Mid-terms draw less voters and the "1s" and "2s" have likely skipped previous mid-terms. Of course, there are exceptions. Young voters weren't registered for the last 5 elections and some people move to California from out of state. I don't know who Capitol Weekly surveyed in their latest poll but I trust them.
Capitol Weekly polled for the primary, asking people whether they preferred Democrat Dianne Feinstein, Democrat Kevin de Leon, or Republican John Cox. Cox is running for governor, so he won't be running for senate. Cox, however, was polled with 9% favorable 6% unfavorable that are 9%/6% with 14% having no opinion and 71% having never heard of him. He's pretty much “generic Republican" to the voters. They could've used almost any name.
In a hypothetical match-up between Feinstein, de Leon, and John Cox, Feinstein gets 40%, Cox 32%, and de Leon 15%. They don’t break down the vote by party but, but Cox’s number is high enough that Feinstein is getting no more than 5-10% of Republican voters. And there are likely a few more Republican points in the 13% undecided voters. If Republicans get 35% of the vote in the primary, De Leon would need Republican voters to split their votes among 3-4 candidates or he’d need to significantly eat into Feinstein’s 40%. This should be a wake up call to California GOP chairman Jim Brulte. He wants a Republican to make the general election. If one is on the ballot he's more likely to get better turn out to help Republicans down ballot. He needs someone with a little bit of name ID and a little bit of money. Neither has to be that big.
The hypothetical general election head-to-head for Feinstein and de Leon has Feinstein winning 36%-17%. In that election 28% said they wouldn’t vote for either. These are likely almost entirely Republican voters. In 2016 15% of voters left the race blank and that was pretty high compared to other states. It certainly would help de Leon in a general election if Republican voters were to leave the ballot blank. If they do vote, Feinstein will probably get 75-80%. Anything over 50% puts him further behind.
This poll is what I expected. There's little path to victory for de Leon due to top two.