I certainly agree that Congress's consent would be necessary under the Compacts Clause. But does anyone really view this as a serious obstacle? They just need to wait for the next Democrat Congressional majority.
As for the constitutionality, I think today's ruling as well as Bush v. Gore make clear that the Supreme Court takes a broad view of a state legislature's power under Article II to "direct" the "manner" in which its "state shall appoint" electors.
The stakes are large: In the last 30 years, the Democrats are 6 and 1 in national popular votes for President. I agree that Republicans might do better if they were actually contesting the national popular vote, but that works both ways: Democrats currently have no incentive to get out the vote and run up the score in deep blue states like California and New York.
Make no mistake. If NPV's proponents can get it passed by enough states to carry a majority of the electoral votes, the NPV compact is happening. The fight will be won or lost in state legislatures. Republicans have done very well in state legislatures over the last couple decades, but they need to keep winning, and they need to act when they do win. For example, if Republicans take control of a state house in a state that has enacted the NPV compact (which doesn't seem likely this year), repealing the NPV compact needs to be a high priority.
One thing to add: Based on my glance of the maps, it looks like all but 3 states where Democrats control both houses of the state legislature has enacted the NPV compact. The only ones that haven’t are Nevada, Maine, and New Hampshire, but it has come close in two of those. The Nevada legislature passed it last year, but the Democrat governor vetoed it. In Maine, the Senate passed it last year, but the House defeated it after a couple reversals.
Passing NPV is clearly a high priority for Democrats in state legislatures. Defeating it needs to be a high priority for Republicans.