  In certain parliamentary democracies, there are two major parties that move
in and out of power every few elections, and a third minority party that
essentially never governs. We present a simple model to account for this
phenomenon, in which minority party supporters sometimes vote ideologically
(for their party) and sometimes strategically (against the party they like the
least). The competition between these disparate tendencies reproduces the
empirical observation of two parties that frequently exchange majority status
and a third party that is almost always in the minority.
