Did the civil war lead to the decline of third party's like the Know Nothing Party, Free Soil Party, and the Whigs which by 1856 really did turn in a third party.
I gather that the US political system was already set into a deeply ingrained trend to bipartitism. Most of those "third parties" had never amounted to much to begin with, though actually without the USCW you could see three big competing political agendas tied to interests of large constituencies in the North, the South and the West respectively.
The problem is that a system with three territorially-based large parties tends to be inherently unstable, as in two out of three will need to come to an understanding of sorts to rule the whole thing.
It may be said to have happened to a point IOTL: the Southern constituency felt isolated without a way out because Northern and Western interests were seemingly in the process to rally towards a Republican party, and the South would be excluded long-term.
It was to some extent a misperception, and it was much more complicated than this, but my opinion (as an imperfectly informed non-American) is that the American political system has significant built-in rails leading towards a fairly stabler two-party system. The war merely accelerated this (and possibly not very much), but it was active well before it.
By the way, I think that a more pluralistic party system would be good for America overall, but very difficult to achieve and requiring large institutional tinkering to be stable. But this bit actually would belong to Chat.