Only... it's not to the advantage of the duopoly to implement something like that, so... entrenched party structure will always be the single choice allowed
Not necessarily — Congress has vast powers under the Fourteenth Amendment. See the just-published "Abolish the Senate" by lawyer and well-regarded liberal commentator Thomas Geohegan [0].
[0] https://thebaffler.com/salvos/abolish-the-senate-geoghegan
[1] https://newrepublic.com/article/158992/biden-trump-supreme-c...
The response to the slippery slope is not to keep on sliding.
Article I section 2 is the relevant part of the Constitution; it makes absolutely no statements about "The states are free to choose their way of allocating their House representatives so long as they are directly elected" and indeed that is absolutely not true; in fact the restriction to single member districts is a federal law passed by Congress, 2 USC 1 § 2c [1], after gerrymandering was producing multimember districts that were not state-wide in order to disenfrancise minorities.
Like, I used to live in Kansas City, Missouri. I used to have exactly one representative I could go to, but his reelection is almost surely guaranteed as long as he wants to keep running. He has run the exact same campaign against the exact same opponent, Jacob Turk, since 2006, besting him by 10-20 points and taking the seat. One of the larger roads across the center of the city is named after Cleaver. With a proportional house I could instead be heard by 8 different folks.
More importantly, in a proportional house my vote can really matter to Cleaver. Maybe he is just a beloved politician and will be the top of the party list no matter what his party is: so maybe his re-election is always guaranteed no matter whether he runs a one-off race against Jacob Turk or not. But he still cares because my vote can be the difference between an extra ally in the House or not. So if I come to him and say “hey I normally vote for your party but we have had a ridiculous heat wave and the scientists are telling us this is our new normal under anthropogenic climate change” he is much more likely to think “oh yeah, I need to care about that because Missouri is being wracked by heat and droughts.”
No, you have always had 3. Your districts congressman, and your 2 state senators.
In geography-based districting, you only have a chance to vote for someone who represents your interests if they run in your district. Gerrymandering is often perpetuated under claims that we need to draw districts to group like voters so they can have representation.
Example: Black majority districts were drawn to allow for Black representatives, but also are used to concentrate Black votes and remove them from other districts. With no districts Black voters instead could still support black candidates across a state and get representation that way. Even better, with multi-seat districts they can vote for multiple candidates for different reasons, including policy; effectiveness; honesty; faith or lack-of; gender, race, sexual-orientation representation; etc...
Proportional representation allows voters to support representatives regardless of proximity, so even smaller minority groups - including minority viewpoints and parties - can effectively pool votes to get representation. If ~10% of the population is socialist, they'll almost never get a socialist representative with districts, but you could expect ~10% representation with proportional representation.
At the same time that concentrating a constituency (such as black voters) into a district will make it more likely they get a representative that cares about their needs, it can also be used to reduce the total number of representatives that constituency would get in other instances.
For example, four districts with ~40% black constituents may be redrawn into one district of 85% black constituents and three districts of 25% black constituents, resulting in a representative that should be more responsive to the black constituents in that district. Conversely, four 60% black districts can be redrawn into two 80% and two 40% districts, resulting in likely fewer representatives that would cater to those constituents and their wants. The exact same strategy can be helpful or hurtful based on the specific makeup of the districts in question.
The problem with the proposed system is that you lose local representation, and you lose a level of definition in the views. If I live in the inner city, do I support the green party, or do I support the group that cares about inner city issues? I'm guaranteed me vote counts towards the group I put it towards, but I may be much less likely to find a group/candidates that actually encompass most my views and wants well (even if that candidate might not have been elected).
These systems aren't better or worse than each other, they're just different, and depending on who they are representing, they will do better or worse. It's entirely possible that if we could accurately measure how well people felt their views were represented and their will carried out a change could result in those going up, and a decade from now a change back to the current system could do the same (likely after a continuous drop in those numbers over years), because the groups and people represented are continuously changing and any assumption that a single system will work best in perpetuity is itself flawed.
It's not like states are some auspiciously chosen grouping of counties with clear benefits.
States exist because the Constitution (and one or two other things) say they exist. The US government cannot pass a law abolishing Missouri, even if everyone really wants to. Missouri, however, can change its county structure, abolish cities, etc. Because the very existence of those forms of government depend on the existence of Missouri.
By design, the US is a federation of more-or-less sovereign States. "Laboratories of Democracy" and all that.
It doesn't really make sense to have proportional representation at any level smaller (or larger) than the individual state.
You can ambush them in a hallway or at a fundraiser, and they'll talk to you, but you'll have just as much luck making an appointment.
The underlying cause is a mistake of our founding fathers. Because of the winner-take-all structure of our elections, our political parties are forced to be forkophobic. The fact that we can't have a real Tea Party, to say nothing of the Greens and Libertarians, causes our civil war proclivities.
A first step is a proportional House. In this proposal, every state stops drawing House districts, which also kills gerrymandering (although gerrymandering by itself does not cause our civil war problems). Instead, parties must register with the state with a list of N names, people in the state vote for the party rather than an individual, and after the election we first off exclude any parties who have not gotten at least 4% of the vote, then allocate the rest by the Webster method that we already use to allocate House seats among the different states. So a party has 10% popularity in California, that means they receive about five California seats, so the top five names off their pre-published California list go on to the House. Forking a political party is now possible, politicians have spines again at least in the House, and Washington starts working again, hopefully.
This probably does not require a constitutional amendment, just a federal law.