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. The 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.

现在生活在美国的人没人能记起民主党和共和党之间的竞争是什么时候开始的。这种竞争进行了一个多世纪，是世界上最早的政治竞争之一。
美国的政治体制是两党制的典型范例。当我们说美国有两党制时，并不是指美国只有两个政党。通常约有十几个政党提名总统候选人。我们之所以称之为两党制，是因为有两个大党派和许多小党派。通常小党派合在一起在全国选举是得票低于全国所投票数的5％。#p#分页标题#e#
在美国社会里，民主党和共和党是最大和最有竞争力的组织。他们通过维持两党制，很轻松地组织选民。因为通常没有其它选择，美国人几乎无可避免地成为民主党员或共和党员。此外，因为这些党的竞争历史很久，多数美国人知道自己属于两党中的哪一派。作为两大党占优势的结果，多数当选官员不是共和党员就是民主党员。在过去的100年里，每年的总统选举中都进行了一些打破旧体制的尝试，但这种体制经受住了所有的攻击继续存在。
两党制是如何能如此坚实地扎根于美国政治中的呢？答案可能就在选举的运作方式中。美国不同于政府议会制国家，我们不仅选举总统，而且还选举许多官员，他们大约有80万人。国会议员也是从每区一票的选举中选出的。例如：我们从435个选区选举435名参议员，每一个选区选举一名参议员。从统计学上来讲，这种选举对大党派有利。选举制度使大党轻而易举地保持着他们的统治地位，因为他们赢得的席位可能比他们应得的份额多。
这种制度的最大结果就是它自动地产生了多数党。因为只有两个竞争者参加竞选，几乎可以肯定其中一个将赢得多数。而且，这种制度似乎有些夸大获胜党的胜利。虽然并不总是如此。但是产生多数党的巨大倾向在这种制度中形成了。
在200多年的宪法史中，美国人对两党制的运作方法已经有很多了解，以使政权能够和平地从一个政党移交到另一个政党。在总统选举中，执政党已被在野党击败了19次，几乎每十年一次。在1860年的选举中，这种政治制度中断了，结果导致了南北战争――美国历史上最大的灾难。历史证明我们有理由对这一制度保持信心，但历史也表明这一制度不是万无一失的。
第二大党能在失败下继续生存，是因为在统计上夸大获胜党的获胜，这能够更有力地支持第二大党对第三、第四、第五党派的对抗。因此，失败的大党能够保持在反对派中的垄断。第二大党相对于第三党派有巨大的优势，它是唯一可能击败执政党的党派，从而能够吸引强烈反对执政党的每一个党派的支持。只要第二大党能够垄断击败执政党的运动，它就是重要的，因为迟早它肯定能上台执政。
两党制的另一个结果是：鉴于小党派可能把自己与特殊的利益和特殊的纲领认同为一，因而有可能采取极端立场，而大党很强大所以倾向于温和。大党温和倾向的证据是许多事务都是超越党派界限进行。当民主党控制国会的一个院而共和党控制另一个院时，会发生什么呢？像一个党控制国会两院一样，虽然一些重要法规可能被搁置，大约相同卷册的法规会得到批准。由于党派的分歧不是根本的，即使在两个党控制不同部门时，政府工作仍然能够进行。