One hundred and five million Americans will vote on Nov. 2, and at least 95 percent of their votes will be cast for the two major parties. For all practical purposes, the Democrats and Republicans have carved up the electorate and left only morsels for other parties, even though many voters express displeasure about not having more options.

Third parties have little or no chance of gaining real representation in Congress or in statehouses. Minor political parties and independents win an occasional seat, but their impact in legislatures is negligible. All this gives rise to a persistent myth about our two-party system, one that is as misplaced as it is widespread: that the United States has always been a two-party country and that there's little to be done about it short of substantial constitutional engineering.

Some say that the dominance of the two parties is caused by the structure of our government and the nature of our electoral system. In particular, it is said, because we do not use some form of proportional representation -- in which representation by parties in legislatures occurs in proportion to the number of votes parties receive from the electorate -- voters do not waste their votes on minor parties. Others point to the genius of the Founders and their system of checks and balances, or to the presence of a vast middle class in the United States that has ensured the absence of deep class animosities that in other countries have led to the emergence of communist or socialist parties.

But the truth is that the United States has not always been so dominated by two parties. Third parties (sometimes even fourth, fifth and sixth parties) once competed successfully in congressional elections, winning significant portions of the popular vote and often gaining seats in Congress. This was true for most of the 19th century and even the early part of the 20th.

Starting in the 1930s, however, minor parties stopped winning significant shares of votes for elections to Congress, and viable third parties in the states have since died away. No longer do Prohibition, Socialist, Populist, Greenback, Farmer-Labor and various Labor parties compete for even one seat. Except for a smattering of minor-party and independent candidates, and a few from the Green Party, Republicans and Democrats dominate our legislatures, the White House and governors' offices, capturing well above 90 percent of the vote.

What happened to eliminate serious third parties? To answer this question, we need to understand why minor parties once drew so many votes. It was because most of these parties had strength in particular regions or even particular states. They were not fully national in scope. Even the major parties had more of a regional flavor than they do today.

Politicians and voters follow power. The decline in voting for minor parties has corresponded to the increasing power of the national government relative to the states. The adoption of a national income tax and subsequent expansion of the federal government with the New Deal created pressures to develop fully national political parties. As the federal government gained more authority relative to the states and localities, voters wanted their votes to go for parties that would have a say in the great national questions of the day, rather than on the issues raised in state or local politics.

As the national government has become more powerful relative to state and local governments, national policies have come to matter more to voters. It's no surprise that turnout is sometimes abysmally low for state and local elections.

Our neighbor to the north provides further evidence of the influence of centralization on the ability of third parties to win votes. Quite a few parties received significant vote shares in the 2004 elections for the House of Commons in Canada. The smaller parties that managed to win substantial votes have their roots in provincial politics, and they drew enough votes from those provincial roots to have a say in national politics. Their success is largely due to the fact that Canada is one of the most decentralized nations in the world.

So if you want to complain about the weakness of minor parties in the United States, don't blame the Constitution or the weakness of unions. Because most policies that determine our economic well-being are made at the national level, we have two dominant, national political parties. Third parties were alive and well in a more decentralized United States, in the days when the states had control over most of the policies voters cared about.

The writers are professors of political science, Pradeep Chhibber at the University of California at Berkeley, and Ken Kollman at the University of Michigan. They are the authors of "The Formation of National Party Systems: Federalism and Party Competition in Canada, Great Britain, India, and the United States."