[boxed]

They aren't forbidden. The voting system is set up so that any new party that enters will torpedo its allies. It's a winner-takes-all and first-past-the-post system, so if a new left party enters and takes 5% of the votes, the democrats will now lose everything. House. Senate. President. All of it.

[RGamma]

That's what coalitions are for.

[boxed]

Again, that sounds great, but that's not how the US electoral system works. You can't transfer votes for your own party to another party in the coalition to gain the presidency. Or to win a district. Or a senate seat.

[bodiekane]

The Republican and Democrat parties are fundamentally coalitions though.

We pretend they represent some sort of Left vs Right split, but really that's a post facto justification or narrative around an arbitrary coalition of sub-factions.

There are a bunch of largely unrelated issues that people care about- immigration, cost of living, abortion, gun rights, crime/policing, foreign policy and various "culture war" issues around race, sexuality, religion.

There is really no reason we group them together as we do. There could be a parallel universe where a liberal "unborn rights movement" grew out of the prior women's rights, black rights, LGBT rights etc and saw abortion as an exploitation of the most vulnerable human rights and said they deserved protection.

There could be a parallel universe where pro-business, pro-market based solution conservatives want completely open borders to allow workers to freely move to work or start businesses where they want.

The parties themselves change over time too. The populist Trump/MAGA faction that has taken over the Republican party has little in common with the "Religious Right" Christian faction or the neo-conservative factions that were influential to the Republicans in the 90s and 2000s.