With no POD restrictions the sky is the limit! You can find timelines on here with the voting system being changed in just about every decade post-1900, with some of the more common ones being around the Progressive Era, the Great Depression, and the Southern realignment. Things you need to overcome:
- Default adherence to the status quo (people need a really compelling reason to care about the change, such as Maine’s frequent election of candidates without majority support recently)
- The two major party’s self-interest (they don’t want more competition, really, but can sometimes be shamed into it)
- Self-preservation for the monied class (it’s a lot easier to corrupt a system in your favor with fewer points of entry)
- Self-preservation for ideas in the political minority (just another subset of money, maybe; lots of people pooling their will and resources because they really care about ideas that benefit from a binary system- think everything from crypto-racism on down to more anodyne stuff like whether we have ethanol in our gas, it’s easier to get your way when your way is only one of two choices)
- Systemic barriers (do you go state by state and get a muddled system? Amendment and deal with the many points of failure there? Backdoor legislation (voting rights and such) and work the problem from various angles and also through the courts?)
Most PODs will offer a legitimacy crisis and a compromise solution, simple as that. Doesn’t have to be very dramatic, but it can be if you like. The system is usually tailored to the nature of the crisis.