Done at the state level is seamless though, nothing changes with the system whatsoever. Not a single thing. The only change is outside of the election system, who the chosen electors cast their votes for.
The change is which electors are chosen to cast the votes. That is most definitely
within the election system. It is the very
function of the election system.
The situation where one candidate wins the popular vote, and a different one wins the electoral college would require some State(s) to ignore the electors that their own voters chose, and replace them with the electors from the other party.
This is a disenfranchisement of the voters in that State.
The 14th Amendment says:
(Bolded part is mine for clarity. The 19th and 26th Amendments supersede the discrimination against women and the voting age language, but the right to vote for the choice of electors remains intact)
"Representatives shall be apportioned among the several States according to their respective numbers, counting the whole number of persons in each State, excluding Indians not taxed.
But when the right to vote at any election for the choice of electors for President and Vice-President of the United States, Representatives in Congress, the Executive and Judicial officers of a State, or the members of the Legislature thereof,
is denied to any of the male inhabitants of such State, being twenty-one years of age, and citizens of the United States,
or in any way abridged, except for participation in rebellion, or other crime,
the basis of representation therein shall be reduced in the proportion which the number of such male citizens shall bear to the whole number of male citizens twenty-one years of age in such State."
