[fireflies_]

It isn't thrown away, though. If your district picks a third party, you send a third party representative to Congress. You'll have different choices in district-specific and state-wide races but you could also have a third party senate candidate and no realistic third party house candidates (for example Maine in 2018, though they had ranked choice voting). I don't see how having multiple levels of elections changes things beyond larger populations making it harder to coordinate outside the parties.

[unsupported]

I stand corrected. Thank you for pointing out my misunderstanding of US elections.

So what is the main difference? size of districts (constituencies) by a factor of ~10? Gerrymandering?