Wikipedia: "A two-party system is a type of party system where only two political parties have a realistic chance of winning an election." This causes problems in a number of ways, one of which is that voters are essentially forced to choose between voting for the lesser of two evils or else "wasting" a vote on a candidate who is unlikely to win.
|The alternate domination of one faction over another, sharpened by the spirit of revenge, natural to party dissension, which in different ages and countries has perpetrated the most horrid enormities, is itself a frightful despotism. But this leads at length to a more formal and permanent despotism. The disorders and miseries, which result, gradually incline the minds of men to seek security and repose in the absolute power of an individual; and sooner or later the chief of some prevailing faction, more able or more fortunate than his competitors, turns this disposition to the purposes of his own elevation, on the ruins of Public Liberty." – George Washington|