It’s not really “established” becaue there isnt any formal body declaring what names different voting systems have.
Are you unclear about what recognition other demcracies give to parties, how there is no prize for 2nd place in America, or why that lack of such a prize gives rise to a two-party system?
While dramatic things like making the senate votes proportional or abolishing the electoral college might require a constitutional amendment, the text is silent on plurality vs RCW or what have you.
Congress could mandate a switch with a simple law, and point to their power to ensure democracy, same as the post bush v gore laws that mandated electronic voting machines.