No one now living in the United States can remember when the contest began between the Democratic and the Republican parties. It has been going on for more than a century, making it one of the oldest political rivalries in the world.

The American political system is a classical example of the two-party system. When we say that we have a two-party system in the United States we do not mean that we have only two parties. Usually about a dozen parties nominate presidential candidates. We call it a two-party system because we have two large parties and a number of small parties, and the large parties are so large that we often forget about the rest. Usually the small parties collectively poll less than 5 per cent of the vote cast in national elections.

The democratic and Republican parties are the largest and most competitive organizations in the American community. They organize the electorate very simply by maintaining the two-party system. Americans almost inevitably become Democrats or Republicans because there is usually no other place for them go to. Moreover, because the rivalry of these parties is very old, most Americans know where they belong in the system. As a consequence of the dominance of the major parties, most elected officials are either Republicans or Democrats. Attempts to break up this old system have been made in every presidential election in the past one hundred years, but the system has survived all assaults.

How does it happen that the two-party system is so strongly rooted in American politics? The explanation is probably to be found in the way elections are conducted. In the United States, unlike countries with a parliamentary system of government, we elect not only the President, but a large number of other officials, about 800,000 of them. We also elect congressmen from single-member districts. For example, we elect 435 members of the House of Representatives from 435 districts (there are a few exceptions), one member for each district. Statistically, this kind of election favors the major parties. The system of elections makes it easy for the major parties to maintain their dominant position, because they are likely to win more than their share of the offices.

One of the great consequences of the system is that it produces majorities automatically. Because there are only two competitors in the running, it is almost inevitable that one will receive a majority. Moreover, the system tends slightly to exaggerate the victory of the winning party. This is not always true, but the strong tendency to produce majorities is built into the system.

In over 200 years of constitutional history, Americans have learned much about the way in which the system can be managed so as to make possible the peaceful transfer of power from one party to the other. At the level of presidential elections, the party in power has been overturned by the party out of power nineteen times, almost once a decade. In the election of 1860, the political system broke down, and the Civil War, the worst disaster in American history, resulted. Our history justifies our confidence in the system but also shows that it is not foolproof.

The second major party is able to survive a defeat because the statistical tendency that exaggerates the victory of the winning party operates even more strongly in favor of the second party against the third, fourth, and fifth parties. As a result, the defeated major party is able to maintain a monopoly of the opposition. The advantage of the second party over the third is so great that it is the only party that is likely to be able to overturn the party in power. It is able, therefore, to attract the support of everyone seriously opposed to the party in power. The second party is important as long as it can monopolize the movement to overthrow the party in power, because it is certain to come into power sooner or later.

Another consequence of the two-party system is that whereas minor parties are likely to identify themselves with special interests or special programs and thus take extreme positions, the major parties are so large that they tend to be moderate. Evidence of the moderation of the major parties is that much business is conducted across party lines. What happens when the Democrats control one house of Congress and the Republicans control the other? About the same volume of legislation is passed as when one party controls both houses, although some important legislation is likely to be blocked temporarily. It is possible to carry on the work of the government even when party control is divided because party differences are not fundamental.