This is not a "compact" in the Article I Section 10 sense, since it's not actually an agreement between states. Instead it's a procedure which doesn't go into effect until enough other states (representing at least 270 electoral votes) pass similar laws. That's perfectly legal and Constitutional.
There are powerful political reasons why this is a futile measure. Most small states and most competitive states are not going to approve similar laws because it would undercut the advantages they now hold in the electoral college. Only large states like California, which are already non-competitive, are going to endorse this proposal because there's no downside -- their electoral votes are already irrelevant in the Presidential calculus.
If we ever approached the 270 electoral vote threshold at which this would take effect, people would start looking very closely at the partisan makeup of the states involved. If going over the threshold would benefit Democrats, then Republican states wouldn't sign on and vice versa. So that 270 electoral vote threshold is a much higher barrier than is immediately obvious.
Think of the following analogy: Suppose California endorsed a Constitutional Amendment to make the number of Senators proportional to a state's population, instead of the present fixed number of two per state. Obviously that would benefit California and other big states. But what are its chances of going into effect? Zero. Because the small states which would be hurt would never ratify it.
Even though this isn't a Constitutional Amendment, it can't achieve it's goal unless the small states and toss-up states go along with it. If large states were to put it into effect, they'd be at an even bigger disadvantage than they are now. So it either won't happen or it will backfire.
Despite that though, it is still unconstitutional without congressional approval.
"This is not a "compact" in the Article I Section 10 sense, since it's not actually an agreement between states. Instead it's a procedure which doesn't go into effect until enough other states (representing at least 270 electoral votes) pass similar laws. That's perfectly legal and Constitutional."
There would still be an implied agreement between all the states giving up all their electorial votes to the popular vote. The way a state who has passed a "similar law" would divide up their electors would be directly conditioned to how many electors participate in this scheme with a "simular law". They might be able to get away with it if it wasn't conditioned to how other states divide up their electors.
The small states cannot stop this. All it takes is for the 13 largest states to pass laws like this. Then, assuming it is constitutional, it would go into effect, without a single small state approving it.
In contrast, it takes the approval of 34 (or is it 38) states to pass a Constitutional Amendment. The large state liberals know they cannot get enough small states to agree to pass a Constitutional amendment to remove the Electoral College. That is why they are trying to get their way though the 'backdoor'. And if they can go 'backdoor' for this, can they do so on other things? How long until 13 big states are making all the decisions for this nations?
How is that not an implicit agreement among the states that pass such a law?
It's an agreement in the same sense that price-fixing is an agreement.
I am not ready to dismiss it. This is a very serioyus and real threat to our Constitution and the electoral college system.