The Constitution guarantees to each state a republican form of government.
Each state legislature has the power to determine how presidential electors are chosen, but that power must be fundamentally consistent with the existence of a republican form of government.
Divesting a state of its power to participate in presidential elections and purporting to transfer that power to other states (through the so-called national popular vote) is not consistent with a representative form of government, but rather is a despotic abnegation of self-government.
The point of all this for the Democrats of course is that the electoral college serves as a bulwark against voter fraud since the vote is by states. Massive vote fraud in Los Angeles can only swing California. They are tired of having to conduct vote fraud operations in Philly, Milwaukee, Fulton County, Maricopa County and elsewhere when they could accomplish the same thing by doing it just in LA or NYC on a massive scale.