The title really says it all. What if the United States adopts/adopted proportional representation? What type of political parties do we see spring up? Obviously the two-party system falls apart almost over night, as there's no longer any reason for big-tent politics.
How does this come about, when, and why? Who supports it, and who is against it?
What parties do we see continue on, and which ones come back from the grave? The Democratic and Republican parties fracture, but does a party (or parties) continue on using their name/legacy?
This is a cross-post with Future History.