"Any manner they choose" has Constitutional limitations. They cannot choose a method that violates other parts of the Constitution. They cannot choose a method that forces other states to give up their own Constitutional powers or protections.
An extreme literalist might say that the Framers never envisioned instantaneous nationwide communications where each state's total vote count were available in near real time, therefore the method of choosing would never have thought to be expressly limited to only the votes within the state's control, given how it took days or weeks to receive news from other states.
Others might argue that the states were sovereign and would never have ratified the Constitution if it included methods where other states can override any single state's Constitutional powers, as the NPV compact does when it disenfranchises the will of the state's voters to the will of larger states' voters.
Another argument is that the compact is unnecessary (and therefore unconstitutional) because the states can already do this without the compact. Any state can choose to allocate its EV to the winner of the national popular vote; they don't need a "gang of 270" to do it. The issue here is political viability of doing so. Any one state legislature is afraid of the blowback of overriding the state's result if it doesn't lead to supporting the winner. They want safety in numbers to ensure the result they want, but that doesn't mean they should be allowed to subvert the Constitution to achieve that.
And the final argument is that choosing a method that results in non-compacting states giving up their own rights is unconstitutional. In this case, the non-compacting states are forced into the compact when their desire is to continue single-state sovereign uncombined Electoral College tallies.
-PJ
Thanks for the thoughtful analysis, and of course I’ll settle for any strategy that effectively stops the NPV gang of 270.
Hey, here’s another thought. What if we could get ONE state, say Alaska, to revert back to having the legislature choose their electors without any input from state voters? Then, effectively, there is no longer ANY presidential national popular vote totals for any purpose.