Popular vote movement is unconstitutional

Brian Messenger:

...
A fundamental problem with the NPVIC is that it is inherently not a republican form of government for a specific state to select that state's Electors. Once a state Legislature decides to ask its citizens their preference through a popular vote, there must be a rational basis as to how the vote of the state's citizens is used to select that state's electors. It is not rational that the people's decision could be overruled by the votes of citizens of unrelated states. The following comparison is between two states in the NPVIC who are at the extremes of the Popular Vote Range for the 2016 election.

Vermont has three electoral votes in our existing system and cast 315,067 votes for president in 2016. This constituted 0.23% of the total votes in the nation. Under the NPVIC, Vermont will give other states 99.77% of the power to select its state's electors for president instead of maintaining the 100% control it presently has. Presently, there is a total pool of 538 electors, and 0.23% constitutes 1.2 electors. Vermont has irrationally thrown away its automatic control of three Electoral Votes for an effective control of 1.2 electoral votes.
...
Legislatures of small states are committing a form of legislative malpractice by joining the NPVIC. The NPVIC is the latest in a 250-year history of schemes where the populous states are trying to bully and dominate the small states in the country. Under the guise of the perceived unfairness of specific presidential election outcomes, the large states are trying to fool the small states into giving up the finely balanced power they were guaranteed when they joined the United States. In addition, as different states implement different rules for voting, all other states would suffer the corruption of the national popular vote by sanctuary states. Those states allow non-citizens to vote in some elections and/or make it likely that errors will result in ineligible people voting in presidential elections.
...

There is much more.

I believe it is unconstitutional for other reasons.  When the Constitution was being ratified the electoral college was devised as a way to encourage the smaller states to join the union.  A popular vote would give far too much power to Los Angeles County which has more voters than around 20 other states.