So a proportional vote interstate compact? :)
I like it - I think one could specify an automatic method for striking a fair bargain between states (and only include states that use that method in the bargain). Then you could have states join the compact asynchronously.
E.g. if the goal is to have the pre-campaign expected electors be the same, and Texas went 18/40 Biden in 2020 while California went 20/54 Trump in 2020, maybe in 2024 Texas assigns all its electors proportionally, while California assigns 49 electors proportionally and the remaining 5 by majority. That would cause the numbers to work out the same (plus or minus a rounding error).
Suppose Connecticut also wants to join the compact, but it's also a blue state. I think the obvious thing to do is to distribute the expected minority electors proportional to total elector count - if Connecticut has 7 electors, it's responsible for balancing 7/61 of the 18 minority electors that are being traded, or just about exactly 2 of them.
But the rounding is sometimes awkward - if we lived in a universe where Connecticut had 9 electors instead, it would be responsible for just about exactly 2.5 minority electors, which is super awkward especially if a lot of small states join and start accumulating rounding errors.
What you could do instead is specify a loss function: you take the variance of the proportion of electors assigned proportionally among the states that are on the 'majority' side of the deal, multiply that by a constant (probably something small like 0.05, but obviously you do some simulations and pick something more informed), add the squared rounding error of expected minority electors, and that's your measure for how imperfect the assignment of proportional electors to states is. Then you just pick the assignment that's least imperfect.
Add in some automated escape hatches in case of change of major parties, change of voting system, or being superseded by a more ambitious interstate compact, and bada bing.
I don't see any reason to structure this agreement as an open-ended compact other states can join instead of a bilateral agreement between just California and Texas as proposed.
(The same reasoning applied to the National Popular Vote Interstate Compact would have its membership closed as soon as they reach a majority in electoral votes, and then completely disregard the votes of any state that didn't sign on, voting in whoever gets the most votes in member states.)
I think it would be better to form a big winner-take-all bloc. With proportional voting, the number of electoral votes at stake will be only a small fraction of the total, so the per-voter influence of CA and TX would probably remain below the national average.
I remember there was a movement a while back to have states agree to award their electors to the national proportional vote winner, but I'm not sure what came of that.
In the USA, the president isn't determined by a straight vote. Instead, each state gets a certain number of Electoral College (EC) votes, and the candidate with 270 EC votes wins.
It's up to each state to decide how to allocate its EC votes. Most do “winner-takes-all,” but some, e.g., Maine and Nebraska, split them up.
California and Texas have the most EC votes of any state, with 54 and 40 votes respectively, so you would think they would get a lot of love from presidential candidates. Instead, they're mostly ignored—California will always be Blue, and Texas Red, so what's the point of pandering to them? This is clearly bad for Californians and Texans as their interests aren't listened to.
So why doesn't California switch to a proportional EC vote split? One that would ensure that there's always something to gain by pandering to Californian interests?
Because most Californians are Democrats, and while a proportional vote split would be good for Californians, it would be bad for Democrats, who in the 2020 election would have lost 20 EC votes—over half of Biden's margin of 37. All in all, not worth it.
But the same issue impacts Texas too, which, if they'd switched to proportional splitting, would have handed Biden 18 votes.
This suggests an obvious solution: California and Texas should mutually agree to split their vote proportionally. That way, neither Republicans nor Democrats significantly gain, but California and Texas would finally be on the presidential campaign circuit!