The only real somewhat serious threat to the US 2 party system was Reform with Ross Perot but that imploded when he tried to bring in Pat Buchanan for some reason and since all third/fourth parties are a joke.

IMO Reform as a serious party died when Perot dropped out in early summer 1992

The Reform Party was only serious as far as Perot's 1992 prez campaign, and that imploded when he suddenly dropped out of the race (later he dropped back in, but weaker)