That means the state which normally would've been a solid block of red now turns purple. Surely that is a compromise of power for those who hold it?

What is the rational incentive to do so?

Any one state is incentivized not to do this alone. It only works if everyone does it at the same time. In addition, if every state did this, states could break away to increase its influence.

That doesn't mean that this is impossible. There seem to be two ways this coordination and prisoner's dilemma problem could be resolved. First, by a constitutional amendment (unlikely) or second in a similar way to the National Popular Vote.

National Popular Vote is a bill with the goal of making the winner of the popular vote the winner of the presidency. The way it works is that states pass a law to give their electoral votes to the winner of the popular votes ONLY until enough states with enough votes pass the same bill.

This bill currently has been enacted by 15 states (195 electoral votes) [1]

[1] https://www.nationalpopularvote.com/

The popular vote has been swinging in the direction of the national Democratic party for over a decade; Democrats would love proportional representation.

Why would the GOP want to give middling members the room to break rank when the margins of power are so razor-thin? What is the selling pitch?

You're shifting the goal posts: is the issue small state power, or republican power via small states?

Small state power is unchanged. Full stop.

Republican power is also not necessarily worse for the reason you gave: blue big states will also be more purple. PR is certainly in the favor of moderate republicans, whoever they may be.

What Ericson2314 is referring to is the coordination + prisoner's dilemma problem. It appears that if every state enacted this at the same time, almost everyone would be better off. That being said, any state that is strongly one party or the other is incentivized not to do this.

California's legislature can choose between giving a high percentage (63.48%[1] in 2020) or 100% of it's electoral votes to the democratic candidate. Alabama's legislature can choose between giving a high percentage (62.03%[2] in 2020) or 100% of it's electoral votes to the republican candidate.

[1] https://en.wikipedia.org/wiki/2020_United_States_presidentia... [2] https://en.wikipedia.org/wiki/2020_United_States_presidentia...

That is very true. This would best happen if the level of government above the elections in question requires they all work the same way. For example, https://en.wikipedia.org/wiki/Fair_Representation_Act_(Unite... would require that all the house districts go SVT PR at once. Likewise, a state could reform all their state legislature districts at once.

I can't wave any magic wand for gridlock at that level, but that bypasses the coordination failure by not requiring coordinated steps at all.