[steenreem]

I think any single-seat election, regardless of voting system, stabilises when there are only two popular parties from which candidates can get elected. I don't believe voting systems such as instant-runoff or approval voting can avoid this problem.

If the US wants more than two colours in their spectrum, they need proportional representation. https://en.wikipedia.org/wiki/Proportional_representation

[dane-pgp]

I'm not sure what you mean by "stabilises". Perhaps you're saying that once the range of choices has been reduced to just two big parties, it is hard to introduce new parties (that successfully challenge the major parties), even if the new voting system prevented the problem of spoiler candidates.

I agree that proportional representation would help shift things, but I don't think it is necessary for the reform to work. For example, it doesn't seem far-fetched that over time, Democrat seats in Congress could be lost to Green or Socialist candidates in some areas, and different factions within the Republican party could also start to win elections under their own banner too.

With some clever campaigning (and/or a lot of money) a popular figure like Bernie or Trump could bring supporters to a "New Left" or "New Right" party that eclipsed the current mainstream parties.