Originally posted by sh76
Another party would take its place in a few years. It would be fun watching various small parties compete for the role of "other" party; but after a while it would make no difference. The "Republicans" (let's say) would simply re-organize under a different banner.
This has happened before, surely. For instance, the early nineteenth-century collapse of the Federalist Party left only one, dominant party, the Democratic-Republicans, but by the 1830s a two-party system had re-established itself. In this case, the decline of one of the older parties occurred before a new party established itself. In Britain in the early decades of the twentieth century, the gradual rise of the Labour Party gradually eroded the base of support for the old Liberal Party, whose traditional voters gravitated increasingly either to Labour or to the Conservatives as class became a more central factor in British elections.
In my opinion, the first-past-the-post system makes it almost impossible for a three-party system to sustain itself - a three-party system could only survive if the parties each had clearly defined geographical and demographic bases of support, so that "No overall control" was the rule rather than the exception. In other words, the situation of three parties vying for power is likely to result in the extinction or marginalisation of one of the three. However, if one side of a two-party system collapsed now, I suppose the result would be an initial period when the surviving party is contested only by independent candidates. An economic crisis or other disaster might ultimately lead to the election of one of these independents (in which case a new party might coalesce around him) or to a breakup of the party into separate factions.