I think something like the interstate voting compact is an extremely deft strategy for approaching a more proportional voting approach for the executive office - for congressional seats there might be a similar approach possible but I'm not certain - such an approach might need to be activated only when a much higher proportion of states sign on - I agree that the leaders would lose if they approached this individually and, honestly, I think we need a really big change to get into a good place - we might get a bit better incrementally.

I'd really like to see the US have regional and balancing seats in congress - the former tied to districts and the latter allocated to maximize the correctness of the representation - so maybe the Purple wins all the races in Maine but Green had 40% of the vote in both races - Green would gain a non-regionally tied house seat[1] while purple gets both of the regional seats.

1. Potentially seats depending on how national elections go

The thing about the Interstate Voting Compact is that it's still winner-takes-all, so as long as a majority of states are on board it's fine.

The problem with doing it with proportional voting is that you need every single state on board; any state still doing winner-takes-all has much greater power.

Yea - but it should still be possible to structure such a compact in a way where you wouldn't create loser states - let's say you target 80% buy-in of states (by representative numbers) before activating and you're talking about the house (the same rule goes for the senate but the activation might fire at a different time) - The only scenario that would really flub the system is if the 20% of house seats not covered by the compact went all to one party (say the purple party) in close races while the green party and yellow party split the vote otherwise - in that case maybe the purple party won 11% of the popular vote and thus is over-represented by the 20% of control they have. Still the other states could balance the math to split the remaining 80% appropriately.

Additionally, as long as that proportion is significantly high enough (say above the 66% mark) the two parties that were partially disenfranchised would still be left sharing a strong majority in the house between them and, if something similar happened with the senate, they could coalition up and force through a congressional amendment that would bring the remaining portion of the country in line for the next election.

The Interstate Voting Compact couldn't be precisely similarly applied to congressional elections, it'd need more momentum behind it, but there is a way for the math to work out.

All that said, IANAL and I have no idea if the legal shenanigans used to make the IVC binding could also be utilized for a similar movement around the house and senate.