But the essential contest is between two parties only. The third, or the third and fourth parties, may secure a limited number of seats, but not enough to become the official Opposition. In 1906, the Liberal Party had 376 seats in the British House of Commons. In 1943, it had 20 seats and that was the beginning in its decline with only 14 seats in 1981.
In April 1981 came to an end the cycle of two-party system when the Social Democratic Party was formed as a result of split in the Labour Party. Social Democrats and Liberals became allies and contested elections together, but it did not make a significant dent on the two major parties. The main contestants are the Conservative and the Labour Parties.
The British Constitution has grown and evolved under the two-party system and its working tends to maintain and perpetuate it. Consequently, the party system as worked in Britain and in those countries that have borrowed her pattern of government implies, strictly speaking, the existence of two parties only.
The essence of party government, as practised in Britain and other countries with two-party system and cabinet government, is that the party enjoying majority in the Lower Chamber forms the government and its leader becomes the Prime Minister.
The Ministers are responsible to Parliament and they remain in office so long as they retain the confidence of the Lower House. On losing its confidence, they are obliged, by a constitutional usage, to resign.
One of the two things may happen then. Either the Opposition party assumes office, if it can command a majority in the Lower House, or, more commonly, the Prime Minister may ask for dissolution of Parliament, when a general election takes place and verdict of the electorate is obtained. The party returned in majority in the Lower House, then, forms the government.
A two-party system, therefore, can be said to exist when:
1. There are only two parties sufficiently strong to share the major part of the electoral vote and to exercise political control, though other parties may exist and obtain some seats in the representative assembly.
2. The major parties alternate in the exercise of power. If this condition is not present as in West Germany where the Socialist Party did not exercise power at the federal level until the 1967 coalition, Japan, where the Liberal Democratic Party dominated post-war politics, till July 1993 elections, although it still commanded the highest number of seats in the House of Representatives.
The lower chamber of the Diet, or in some parts of the United States of America where one party traditionally wins, the regime cannot exactly be termed a two-party system. “Perhaps,” says Michael Curtis, “the term ‘one and a half’ party system might be applicable in such circumstances.”