This is a brief rundown of the 1993 election the last we had under FPTP.
https://en.m.wikipedia.org/wiki/1993_New_Zealand_general_election
Both main parties were starting to unravel. Also has an interesting electoral map. Each electorate was roughly the same size in terms of population. 4 parties in power but in effect two as the other 2 had 3 seats combined.
Electoral reform took around ten years. I think in 81 a third party got around 20% of the vote and 1 seat.
Referendum, both parties agreed to it due to collapsing party membership. Party dues were a major source of money back then plus boots on ground in electorates.
Corporate money is a big problem, they shouldn't be able to donate or if they can it goes into an electoral slush fund for election costs.
Rather than corporate dollars the parties would need members to donate time and money.
Money helps here Labour won despite being outspent 3-1.
If the GoP does collapse and keep doing it eventually they're going to need votes from place like New York and California.
Note liberals seem to think proportional representation will help them. Our governments have been 50/50 since 1996, it drags things towards the centre it locks the hard right and left out.
Minor parties still get dominated and have to make the choice of supporting center right or left parties. If they campaign on changing the government then support it their votes tend to collapse.
You would probably have 4 or 5 parties emerge in the states if you had proportional representation. Trump could still have won he would have done some rallies in New York or California.
Well you would probably have more parties than four or five but subsequent elections would think them out since they would probably be founded by charismatic members of Congress in safe seats loyal to them over the national party.
1987 went from 2 parties to 6 in 1996 which thinned them to 5 after the election we're down to 4 parties now with plus one hangover left from 1996 due to being gifted an electorate seat (0.5% popular vote).