At least two, that I can think of. One to change the way representatives are elected, and one to amend/overturn the 17th Amendment to change how senators are elected.
I’m pretty sure that unless you mandated that all state governments also be proportionally represented (an insurmountable hurdle, as that’s not within the Federal government’s purview in the least bit), you’d still effectively end up with a two party system, as all the candidates would still be generated out of the state-level two party system.
I mean, who’s Joe Six-Pack in Ohio going to vote for in a Congressional election? The Republican candidate who used to represent part of the state in the Legislature, or some clown running for Congress from the Socialist party, who has no foothold and no money in the state?
About the electoral college. Apparently it was originally conceived as a kind of nominating convention to choose three candidates, with the final choice made in the House. With each state having one vote. Almost like the way parliaments choose prime ministers. Parties were not anticipated by the founders. You can see how well that worked. It is time to rethink the whole concept. You can imagine how well that is likely to work.
There was a political rearrangement in Canada around 2000. The so-called Progressive Conservative party was reduced in 1995 from a majority government to exactly 2 seats. It continued to operate for a few years while a new Reform party formed in Alberta (aka Texas North). While not exactly the same as the Tea Party, they tended in that direction. They eventually absorbed the empty hull of the PCs and renamed themselves the Conservative party (no progressives need apply), won a couple elections and governed for a dozen years until replaced nearly a year ago. They were typically tea party like in one respect: they didn’t want their policies to be based on facts (such as global warming).
My idea had only proportional representation in the House, not the Senate. If you wanted it in both houses, then, yes, you’d need to change that too.
But in the Congressional race, he wouldn’t be voting for any single person, just the party. It’d be up to the party to make sure their representatives are from a variety of geographic areas if they felt the need to appeal to the electorate that way.
As far as states adopting this, if the national government does it, I expect a lot of states would copy them. Consider that virtually every state has a carbon copy of the federal government now.
I would guess that any move toward a proportional representation (PR) system is more likely to start at the state or local level and then, if it works, move up to the federal level.
One way it could work would be for each major party to have “caucuses” in the PR states that would run in the local races like they were different parties. The politicians would still be a part of the same parent party, and would support that party at the fed level.
For instance in this imaginary setup, I would be running in the local district for the state assembly (in a district that gets multiple seats*) as a Democrat, Freedom caucus. My neighbor is running in the (D), People’s caucus. There could also be an ®, Christian caucus, an ®, Libertarian Caucus.
When it comes time for the federal house race where each district gets one seat, me and the People’s caucus person could each be a Dem in the primary, and the non-winner would presumably support the winner in the main race.
*Most districts would probably need to have more than about 5 seats for this many “parties” to be a likely outcome.