No, this is a proposed compact between the states. It only has to be approved by each state. Each state has the right to allocate its electoral votes anyway it sees fit. The proposed compact can be entered into and withdrawn from by its member states.
"Besides California, legislation to create a compact was introduced this year in Colorado, Missouri, Illinois and Louisiana. Proponents are pushing to have similar bills in all 50 states next year."
This is an end run around the electoral college, which would require a constitutional amendment to change. This compact would give the bigger states more power to determine the outcome.
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This is an end run around the electoral college, which would require a constitutional amendment to change.
Abolishing the electoral college would require a constitutional amendment. But as you point out, each state has the authority to allocate its electoral votes as it sees fit, and if each state votes to do so in proportion with the popular vote, it's constitutional.
Personally, I'd go for the system of allocating one electoral vote per congressional district and two to the winner of each state at large. That would reduce the possibility of a risk of a majority candidate winning the presidency while still preserving one function of the EC, which is to turn a plurality into a majority. This compact would give the bigger states more power to determine the outcome.