That's not the case at all.
In Germany they have one Land (state) that has 21% of the population.
They do not dominate politics in Germany.
Essentially in Nordrhein-Westfalen 32.6% voted for CDU (equivalent of the Reps), 24.2% voted SPD (equivalent of the Dems), 9.4% for the AfD (like MAGA/Tea Party), 10.4% for the FDP (like center right), 7.5% for die Linke (a further left party), 7.6% for the Green Party.
So, in the region people vote for what they want. And what they want is far more diverse than a two party system could ever decide.
My prediction would be that if PR entered the US for the House and Senate, that there would be a diversity of political parties trying to get votes from people all over the country. It wouldn't be about certain areas.
Some areas might see more of one party than other areas, but in reality there wouldn't be any control. You'd see political parties joining together to form coalitions which would lead to more cooperation between parties.
Also voters would have more oversight. You can see how quickly people get fed of political parties. The AfD rose in a few years to get a lot of seats, a similar party in the UK (with FPTP like the US) it took them 30 years to get one or two seats.
