US stability depended partly on the structure and process established by the constitution.
But it also depended on the extra-constitutional party system, especially the mostly one-party Republican rule from Lincoln through Hoover and the Democratic rule from FDR through Carter.
https://en.wikipedia.org/wiki/Party_divisions_of_United_States_Congresses
The party structures allow a small fraction of the population to control the political outcomes.
“The party structures allow a small fraction of the population to control the political outcomes.”
Uh??
Every country, except dictatorships, have political parties. And it’s absolutely not the case that it allows a “small portion” of the population to control the outcome - especially in the US,which has a winner take all system.
You need to have a majority to gain power at the various level of government, and a majority is not a “small portion”.