The whole scheme incentivizes voter fraud in all the big cities--especially in states which are reliably Democratic. Before it could be taken for granted that the Democrat would carry the state so no strong reason to manufacture a lot of fraudulent votes. Now the fraudulent votes could swing the national outcome.
Such a compact becomes the equivalent of federal law enforced by federal courts. Changing the rules AFTER an election is a big no-no. Ask Al Gore.
Urban areas are a larger percentage of the population every year, the leeches therein provide the Demons with their instruments.
How is this to be countered? Chicago has not had a Republican mayor in 75 yrs.