What it boils down to is states giving their electoral votes to CA, which produced Clinton't entire margin of "victory" in 2016. Unfortunately the blue states seem entirely on board for that.
As I've said before, one problem is the idea that one state's current legislature and governor can change the rules on the fly, maybe even after election results are in, potentially reversing the outcome in either direction.
Another issue for the courts to ponder is that states joined the union with certain assurances as to their rights and roles in federal elections - can that be circumvented without (or even with) a constitutional amendment? I hope not but who knows?
Edit> Oh and let's not forget the issue of voter fraud. It's probably already rampant in CA but currently doesn't affect the net result of their presidential vote. Change the rules so those votes do make a difference and institutionally encouraged fraud will skyrocket.