During the late 19th and early 20th centuries, Spain was governed under a system known as el turno pacifico, “the peaceful turn.”
Through a bit of electoral jerry-rigging, control of the government periodically alternated between an official “Conservative” party and an official “Liberal” party. Alternatives were locked out and political activity was channelled into this narrow and strictly linear right-left spectrum.
To make a long story short, the eventual result was that the pot boiled over, the communists and fascists started shooting at each other, and the country fell apart in a chaotic bloodbath.
I wish American politics didn’t remind me of this series of events as often as it does. Official two-party-only politics is a ticking time bomb of perverse incentives and Manichean thinking.