And the parting on the left
Is now parting on the right
And the beards have all grown longer over night
There is talk among Republicans dissatisfied with developments in the current presidential campaign of forming a 'third party.' Implied they mean significant third party as there are, and have been, numerous other obscure political parties that have placed candidates on ballots.
Personally, I applaud the idea of numerous (i.e., beyond three) political parties as the competition should suppress markets for political favor.
I am not well-versed in political theory, but I suspect there has been research that demonstrates that, in a system governed by a decision rule that says that candidates with majorities of votes win (a.k.a. 'democracy'), then the tendency over time should be for only two significant political parties. This would seem particularly true when the system permits the winners to employ the strong arm of government to forcibly take resources from the losers for their benefit.
In the spirit of game theory, coalitions of special interests are likely to form to amass votes for their side. Because the goal is to get just over 50% of the vote, it seems that no coalition would be satisfied with, say, 5% of potential voters because they know that they would lose the game. Instead, special interests would keep combining until they get to what they think is a winning mass--about half the vote. Third parties would be 'acquired' or crowded out into obscurity.
In democracies, there seems little theoretical room for a third significant party. This suggests another way how democracy and diversity are opposed.