Two points regarding the possible downsides of proportional representation and multiparty systems:
First, different ways of implementing PR lead to different numbers of parties. If you just do nationwide party list, without a threshold like requiring at least 5% support to win any seats, there will be tons of parties, and the wheeling and dealing between them will ultimately seem more impactful than the actual election. But if you have multi-winner districts with < 10 seats each, only a few parties will win representation (but probably more than two). There’s an important sense in which politics is compromise. With a two-party system, voters have to compromise and support the more tolerable of two candidates. Then, whichever party wins a majority gets to govern uninhibited by the other party (aside from filibusters, the possibility of a split congress, etc.). Or with a 10+ party system, voters hardly need to compromise at all; with so many options, chances are they’ll be able to find a party they absolutely love. But this doesn’t remove the need for compromise, it just shifts it onto the elected officials. I think the optimal system splits the difference, putting some of the weight of compromising on voters and some of it on elected officials. Having more than two parties, but not more than six or so, should achieve this. To the extent you’re worried about the effects of having significantly more than three parties, this is entirely manageable with multi-winner districts—but I think your points have a fair amount of validity even for a three-party system.
(An additional consideration is that the US has a directly elected executive branch, so you wouldn’t necessarily have the dealmaking to determine who gets to form a government that you get in most places that use PR.)
Second, many of the harms of polarization are different from what you describe happening under PR systems. I don’t think the risk of a civil war or democratic backsliding is driven by a handful of extremists who need to be excluded, it’s driven by the widespread hatred that Democrats and Republicans have for one another and the perceived threat the other party poses to democracy. I think this has relatively little to do with the difficulties of dealmaking and controversial policy concessions to fringe parties, even though I lumped the risk of civil war and congressional deadlock together as “polarization”. In my model, I assumed that electoral reform would affect all the consequences of polarization equally. This could be very wrong! Perhaps proportional representation eliminates most of the congressional gridlock caused by zero-sum partisan maneuvering, and replaces it with congressional gridlock caused by the difficulty of getting several different factions to work together. (I find Lee Drutman’s argument from Breaking the Two-Party Doom Loop that having more parties will result in a more functional congress to be quite persuasive, however.)
Two points regarding the possible downsides of proportional representation and multiparty systems:
First, different ways of implementing PR lead to different numbers of parties. If you just do nationwide party list, without a threshold like requiring at least 5% support to win any seats, there will be tons of parties, and the wheeling and dealing between them will ultimately seem more impactful than the actual election. But if you have multi-winner districts with < 10 seats each, only a few parties will win representation (but probably more than two). There’s an important sense in which politics is compromise. With a two-party system, voters have to compromise and support the more tolerable of two candidates. Then, whichever party wins a majority gets to govern uninhibited by the other party (aside from filibusters, the possibility of a split congress, etc.). Or with a 10+ party system, voters hardly need to compromise at all; with so many options, chances are they’ll be able to find a party they absolutely love. But this doesn’t remove the need for compromise, it just shifts it onto the elected officials. I think the optimal system splits the difference, putting some of the weight of compromising on voters and some of it on elected officials. Having more than two parties, but not more than six or so, should achieve this. To the extent you’re worried about the effects of having significantly more than three parties, this is entirely manageable with multi-winner districts—but I think your points have a fair amount of validity even for a three-party system.
(An additional consideration is that the US has a directly elected executive branch, so you wouldn’t necessarily have the dealmaking to determine who gets to form a government that you get in most places that use PR.)
Second, many of the harms of polarization are different from what you describe happening under PR systems. I don’t think the risk of a civil war or democratic backsliding is driven by a handful of extremists who need to be excluded, it’s driven by the widespread hatred that Democrats and Republicans have for one another and the perceived threat the other party poses to democracy. I think this has relatively little to do with the difficulties of dealmaking and controversial policy concessions to fringe parties, even though I lumped the risk of civil war and congressional deadlock together as “polarization”. In my model, I assumed that electoral reform would affect all the consequences of polarization equally. This could be very wrong! Perhaps proportional representation eliminates most of the congressional gridlock caused by zero-sum partisan maneuvering, and replaces it with congressional gridlock caused by the difficulty of getting several different factions to work together. (I find Lee Drutman’s argument from Breaking the Two-Party Doom Loop that having more parties will result in a more functional congress to be quite persuasive, however.)