Two-party systems are states such as Honduras, Jamaica, Malta, Ghana and the United States in which there are two political parties dominant to such an extent that electoral success under the banner of any other party is almost impossible. One right wing coalition party and one left wing coalition party is the most common ideological breakdown in such a system but in two-party states political parties are traditionally catch all parties which are ideologically broad and inclusive.
The United States has become essentially a two-party system, since a conservative (such as the Republican Party) and liberal (such as the Democratic Party) party has usually been the status quo within American politics. The first parties were called Federalist and Republican, followed by a brief period of Republican dominance before a split occurred between National Republicans and Democratic Republicans.
The former became the Whig Party and the latter became the Democratic Party. The Whigs survived only for two decades before they split over the spread of slavery, those opposed becoming members of the new Republican Party, as did anti-slavery members of the Democratic Party. Third parties (such as the Libertarian Party) often receive little support and are very rarely the victors in elections.
Despite this, there have been several examples of third parties siphoning votes from major parties that were expected to win (such as Theodore Roosevelt in the election of 1912 and George Wallace in the election of 1968). As third party movements have learned, the Electoral College’s requirement of a nationally distributed majority makes it difficult for third parties to succeed.
Thus, such parties rarely win many electoral votes, although their popular support within a state may tip it toward one party or the other. Wallace had weak support outside the South. More generally, parties with a broad base of support across regions or among economic and other interest groups, have a great chance of winning the necessary plurality in the U.S.’s largely single-member district, winner-take-all elections. The tremendous land area and large population of the country are formidable challenges to political parties with a narrow appeal.
The UK political system, while technically a multi-party system, has functioned generally as a two-party (sometimes called a “two-and-a-half party”) system; since the 1920s the two largest political parties have been the Conservative Party and the Labour Party. Before the Labour Party rose in British politics the Liberal Party was the other major political party along with the Conservatives.
Though coalition and minority governments have been an occasional feature of parliamentary politics, the first-past-the-post electoral system used for general elections tends to maintain the dominance of these two parties, though each has in the past century relied upon a third party to deliver a working majority in Parliament. (A plurality voting system usually leads to a two-party system, a relationship described by Maurice Duvergerand known as Duverger’s Law.) There are also numerous other parties that hold or have held a number of seats in Parliament.