What would it take for Proportional Representation to become the predominant method of selecting legislative seats in the United States? I'm throwing it out there because while it's interesting to theorize a PR-style government in the U.S., given how unlikely it seems either major party would sign its own death warrant, I'm not sure anyone has come up with a plausible means of electoral reform.
As a general primer, in the U.S. without substantial changes to the constitution, it would be impossible to alter how the President or the Senate was elected. The House can constitutionally be elected by PR, although statute currently prohibits it (since the 1950s, IIRC). If there was no limiting statute, each state could independently set up a proportional representation electoral system for selecting its own representatives, although this would be useless for states with only 1 representative, and presumably most states with two or even three reps would still elect almost entirely Republicans and Democrats.