Trump got 4,483,810 votes in CA alone.
That's more votes for Trump in CA than the entire populations of 25 states (Wyoming at #50 to Kentucky at #26).
Image now if CA succeeded in putting this requirement in place. A state like NY could follow (probably not in time for 2020, but for future elections to hurt (R) candidates). In Trump's case, that was 2,819,534 votes in 2016.
Just those two states, could have prevented 7,303,344 votes for Trump. Throw in WA, OR, MA and other liberal states, and that number only gets larger.
Trump wins the electoral college, but looses by 10 - 12+ million popular vote.
Take that message to the politically, constitutionally & civically ignorant masses in this country, and a groundswell to remove the electoral college from our Presidential elections could pass the numbers needed to amend the Constitution.
This should not be looked at in isolation in CA alone, nor just for the 2020 election.
Maybe in 2024, the (D) candidate doesn't want their tax returns made public and the (R) candidate doesn't want their [college records, medical records, etc] made public. These liberal states could then remove the tax return requirement law, and put in a college records law for that election.
Allowing custom requirements like what CA tried to do could be a very powerful tool, to be combined with all the other tools the libs have (media, schools/universities, Hollywood, etc), to keep (R)'s off the ballots of numerous states in the union.