Replying to you again, very late.

> Fortunately we can look to other countries that have managed to avoid political duopoly by using voting systems which don't penalize people for voting for new parties. Even better, some US states have already implemented such a system[0], and, going back to your point about constitutions, the people of Maine managed to introduce RCV not because of a constitutional requirement, but despite a narrow (state) constitutional prohibition.[1]

I think you are right, voting systems are the best place to start.

That and prohibitions on justices and congressfolk from weighing in on matters they have a personal or political interest in, such as receiving political funds or assistance.

Imagine if donating to a politician whose influence you want will make it more likely they cannot help you. That leaves donations reflecting people's assessment of who will better run the country in a more general sense. Those donations look more like "free speech" than the rampant influence purchasing that overwhelms the system today.

> The federal constitution says that voting is left up to the states, so arguably the centralization problem is ~50 different experiments which have all gone wrong in the exact same way

If the constitution forbid parties from operating across state lines, or operating in more than 20 states, or holding more than 20% of seats in either Federal legislative body, … there would never be single party rule at the Federal level which is where it would matter most.

> If both major parties split into 50 different organizations that all happened to endorse the same candidate for president (but for nominally different state-specific reasons), should the SCOTUS have the power to ban those political organizations (and perhaps ban one and not the other)?

While collusion (meaning surreptitious coordination, in this case) between 50 state level parties would certainly be possible, it would be substantially more difficult than coordinating 50 state offices of a single political party as it is today.

Today, all senate campaigns are basically running for one shared constituency: big money from anywhere in the US, and political support from the same party, across the US.

The tight link that should exist between a politicians power base and the politicians electorate has been broken, for state level elections of Federal positions.