I actually think the US hard two party system isn't purely due to first-past-the-post, it is actually due to four interacting factors: (1) first-past-the-post;

I actually think the US hard two party system isn't purely due to first-past-the-post, it is actually due to four interacting factors: (1) first-past-the-post; (2) presidentialism; (3) a deeply-entrenched primary system; (4) cultural homogeneity Point (4) will likely strike many Americans as dubious, but the US doesn't have anything comparable to Québec – there is presently no US state where English isn't the majority language; nor anything comparable to Scotland or Northern Ireland (no state where a significant percentage of its population identify with a competing national identity). So, no matter how much (undeniable) diversity the US has, in the sense that counts for this purpose, it doesn't. Hence: while abolishing first-past-the-post would no doubt do a lot to break the US hard two-party system, it could also be broken while retaining it if something happened to (2) or (3) or (4) instead. But, in US politics, changing (2) or (3) is even more politically fringe than changing (1) is. Maybe that might change at some point though, who knows what the future holds. And (4) will likely eventually cease to be true, but only in the long-run in which we are all already long-dead.