Isn't it likely that the popular aspects of a third party's platform just gets absorbed into one of the major parties or both? We've had two parties dominating for 160 plus years so there hasn't been a major party collapse for a long, long time.
The post-revolution period and antebellum years were so unique in that regional sectionalism was sharply defined and at the forefront of U.S. politics.
Yes, exactly.
There is a reason people refer to this as the Sixth Party System - because every so often there is enough of a shakeup that what the parties stand for and where they are strong changes radically.
But as you say, the structural forces push things toward the party having these coalition fights internally in the primaries and so on rather than afterwards. When an issue the general public is interested in isn't being addressed, it can bubble up into a third party that is briefly mildly competitive and then those policies get absorbed into the major parties (either one or both). Slavery was sufficiently huge as such an issue that it took down the Whigs and the Republicans took their place. (After a period of chaos.)
That's the thing about the call for a third party, people have to make a case for where it is going to get its votes and from who. And if there
is a good case, it just means the most likely result is one of the other two parties just adopting those positions after the vote split costs them an election.
It's also true that the regional aspects have been blunted even more since the parties got more ideologically consistent, so you don't even have that option to grow some third party strength.
The fact that it is a presidential system not a parliamentary one doesn't help. In Canada, we have parties that exist purely because of regional strength, but that's not really supported well in the US system.
We haven't even seen a third party try to organize a purely congress-level push in my lifetime. They just come out for the Presidential elections, where they have the least chance of accomplishing anything long term.