I’ll state up front that I’m certain there are strong reasons why the US doesn’t have state specific political parties that are distinct from national parties. It is a mystery only in so far as there are limits to my understanding of the history. However, having said that I was just generally thinking about third parties in the US and one way first-past-the-post systems can still move beyond a two-party system is with very regional parties. So takes this as me, as per-usual, thinking out loud rather than telling-it-like-it-is.
In the UK this manifested historically in Scotland, Wales and Northern Ireland. The importance of parties like the SNP or Plaid Cymru has been increased in recent decades by devolved governments and a more federated system of government (conditions which have always applied in the US states). Yet, even prior to the reforms that set up a Scottish parliament, the UK parliament had some third party MPs from regional parties. The political party situation in Northern Ireland was different again with both Labour and the Conservative Parties not operating there (although the SDLP is an equivalent party to Labour).
So the US has some of the features that would enable region-specific third parties within a two party system. There’s arguably even an advantage for states to have a senator from a state-specific party IF the senate is otherwise roughly evenly split between the two national parties. A non-aligned mp/senator/representative can be paradoxically more powerful in a chamber when they end up being a deciding vote. They are less beholden to party loyalty and they can more openly state that they will vote with which ever party on the day is offering a better deal for their state/region. That would suit some of the claimed anti-ideological populism (often just species of conservatism) which has a long history in the US.
So why does the Repub-Dem split seem to be operate at multiple levels? There’s zero mystery why the Presidential election is wholly inimical to third parties — everything about it is geared towards a stand off between two candidates and those candidates need national parties behind them. The electoral college system means those parties have to operate at a state level and need state-level political apparatus. The process of picking the national candidate mirrors the state-by-state aspect of the electoral college, further necessitating state level version of national political parties.
However, that alone isn’t a sufficient explanation. The UK’s national parties operate in Wales and Scotland but regions still create a scope for regional parties. Geographic based electoral systems rely on the existence of local majorities, i.e. geographically concentrated support to pick candidates. So there is a built-in capacity for regional issue candidates to gain some third party traction in a way that broader issue candidates (say a Green Party) can’t without electoral reform.
So what else is in play? In the UK historically the Scottish and Welsh nationalist parties have been specifically nationalist in character, all be it in a more progressive left-of-centre way than the term ‘nationalism’ may suggest. That nationalism rested on historic, cultural and linguistic differences as well as an aspiration for independence. Although there is periodic discussion about Texas seceding form the USA or California, there’s not really the deep national identity for US states that is anything like that of the non-England parts of the UK. Having said that the Liberal Party in its various forms remains a significant third party in the UK after the the two party system shifted to Lab-v-Con around WW2, partly by being a regional-party-lite within England.
Additionally, national politics and the pressure of the electoral college vote strategies has tended to sort US national politics into Red-v-Blue regions where the Republican (say) states have a degree of similarity in terms of demographics (more rural, less dense, less urban) and likewise for the counterparts. Each state may have their own issues but the differences between a given state and another similar state aren’t sufficiently large that they can’t be accommodated within two broad coalitions of interests.
Another aspect that has taken me much longer to understand was the extent to which the two main political parties have their own local character. This is obvious historically with quite severe differences within the Democratic Party in the past, arguably greater within the party than between the two parties. That’s not quite how other countries think of political parties.
Another thing that it has taken me longer to understand because of preconceived ideas from British politics, is the degree to which US Senators are more independent actors than parliamentarians in the Westminster system. It’s rare (and indeed a sign of a government in severe trouble) in the Westminster system when there is any doubt about how a non-third party representative might vote given their party’s position. That’s not the case, as I understand it, at least in the senate (I mean, technically its not the case in the UK’s House of Lords but that is such a messed up concept for a second chamber that it defies any sensible analysis — in the Australian Senate, Labour senators vote Labour and Liberal senators vote Liberal and it would be a massive deal if they didn’t).
No final conclusion, as this was just another episode of Camestros trying to answer his own question. Still, if somebody gave me a gazillion dollars to make the US a multi-party system, I would definitely start regional.