Apart from such trifling low hurdles as the vested interests and absolute power of the Republicrats, I see one basic obstacle to introduction of PR in America:
Nobody talks about it and nobody knows what it is. (And try explaining it on a bumper sticker.) At various times I have asked several major-party candidates for public office (including Alex Sink, Democratic nominee for governor of Florida, recently) for their opinions on PR. They didn’t know what it was – not the phrase, and when I explained, it was clear they were not familiar with the concept either. Some (not Sink) at first thought “proportional representation” was about representation of minorities through racial gerrymandering. Even minor-party candidates usually need it explained. I made a presentation on PR to a local Reform Party chapter once, and they seemed to remain perplexed (and I got a definite vibe from the discussion that some thought of PR as a way of giving more representation to racial minorities, some liberal thing for which they were not enthusiastic). Those minor-party candidates who did know about it, such as Ralph Nader, never mentioned it in their speeches; but all who did know about it were for it.
PR does have a history in the U.S. Some cities tried it in the late early 20th Century; the Progressives were for it, as a way around the power of the urban political machines. But it was abandoned in the late 1940s or, put it another way, the powers that be put it down. Since then, it has not been part of our public discourse, not even in the 1960s or '70s, and most Americans don’t seem to have ever heard of PR any more. Even though most of the world’s democracies use it – almost all except for France, Britain, and Britain’s former colonies, including the U.S.
The American media paid no attention when New Zealand moved to the mixed member proportional form of PR in 1996. Why would they? But if the UK does it, they’ll have to cover that. And then, maybe, just maybe, just a bit of public attention on it will ease PR onto the agenda for serious public discussion in America.
And once there, in the Information Age, it will stay there a long time.
But, IMO, to be successful, a PR movement would have to be a trans-ideological movement. All the minor parties – Libertarians, Greens, Constitution, America First, Socialists, Communists, Tea Party – would have to get under one big single-issue tent and work together amicably, even if they are worst enemies on every other issue.
That would be fun to watch.
That would be fun to do! 
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