The Democratic and Republican parties are the largest and most competitive organizations in the American community. 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 political 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 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. 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. In over 200 years of constitutional history, Americans have learned much about the way in which thepolitical 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. According to the passage, what's the fundamental reason for the coexistence of two major parties?