Hardcore harvesting
Actually, it's not true that "we have no way of knowing if those are Dem or Rep ballots". The California ballots are all designed to be mailed, and whether you mail it or drop it in a drop box, the instructions say you must put your name, address, and signature on the candy pink envelope.
That makes it a simple matter to compare each ballot to the public voter registration lists, and make all but the Republican ballots disappear.
Easy peasy.
BTW, I turned in my ballot Tuesday, at City Hall. And even in such a supposedly secure location, I examined the drop box carefully to make sure it really was put out by the county elections office. We live in creepy times.
Can you guess who I voted for for President?