It is anyone’s guess what the parties in a House elected by PR would be. (The Senate would continue to be elected the way it is now, and such effect as there would be on it would be indirect.) Although voters might continue to identify with the two major parties for a time, dissidents would soon learn that third-party votes were not wasted.
The largest party in the House might well be the Republicans. As it is, their party, with its homogeneous and stable group of core voters and centralized, disciplined organization, is far more like a European party than the Democratic Party is. If PR were adopted, the Republicans might lose their right wing to a new conservative party or parties, but the number of right-wing Republican voters is fairly small (as the Patrick Buchanan campaign unintentionally demonstrated, by adding only single-digit figures to large protest votes). The Republicans might compensate for their loss by becoming a more consistently classical-liberal party–pro-business, pro-choice–and attracting fiscally conservative social liberals who now identify with a Democrat like Paul Tsongas. Such a neoliberal Republican Party might hold steady at 35 to 40 percent of the House.
The Democratic Party, an incoherent coalition of smaller proto-parties, lobbies, interest groups, and machines, which are brought together only by the winner-take-all logic of our electoral system, would, however, probably disintegrate. The breakup of the Democratic Party as the result of PR would not mean that the power of today’s Democratic voters would decline. On the contrary, the kind of moderate Democrats represented by the Democratic Leadership Council, freed from the electoral necessity of appeasing ethnic and liberal lobbies, might well prosper. Together with “Reagan Democrats” wooed back from the Republican presidential coalition, moderate Democrats in Congress might form a Populist Party equivalent to Christian Democratic parties in Europe. Slightly right of center on social issues and foreign policy, slightly left of center on middle-class benefits, such a party could be expected to draw substantial support from Northeastern ethnics, middle-and lower-middle-class whites and Hispanics in the South and West, and perhaps socially conservative blacks. It would be heavily Catholic. A Populist Party might be the nearest rival to the Republicans for the status of largest party in the House.
The United States, unlike Europe, probably would not have a strong Social Democratic Party, given the low level of unionization and the lack of a mainstream socialist intellectual tradition here. There might nevertheless be something that called itself the Social Democratic Party, representing unions, farmers, public-sector employees. Heavily black and Hispanic, such a party would favor protectionism and government subsidies to industry.
A small American Green Party would almost certainly arise from the decomposition of the Democrats. Appealing to New Age environmentalists, pacifists, feminists, and gay-rights activists, such a party might have trouble winning five percent of the vote in successive elections. So might other fringe parties that are easy to imagine: the Conservatives, a far right-fundamentalist alliance; the Multicultural Coalition, a coalition of ethnic-separatist parties; and anti-tax Libertarians.
Carrying this speculative exercise one step further, we might assign percentages of House membership to these hypothetical parties. Based on European experience and American political subcultures, the pattern might be as follows: Republicans (40 percent), Populists (30 percent), Social Democrats (15 percent), Greens (5 percent), Conservatives (5 percent), Multiculturalists and Libertarians (5 percent between them).
One question that immediately comes to mind is, Would a multiparty system exaggerate the power of small, extremist parties in the House? The danger that such parties might hold the key to House leadership would be eliminated by the leadership reforms proposed above. If the position of speaker of the House rotated among the members of a leadership committee elected by cumulative vote, moderate parties would be assured of exercising power in their turn and having substantial weight on the leadership committees, without having to make any compromises with extremists.
The chances that tiny fringe parties would win significant representation would be limited by the division of the country into multimember districts. The danger of extremism could also be guarded against by setting a threshold for participation in the legislature. Germany’s requirement that parties get five percent of the nationwide vote, for example, has been enough to exclude both the neo-Nazi Republicans and the radical-left Greens. Anyone who argues for a higher threshold should explain just which 10 or 20 percent of his fellow citizens are so dangerous that they should be effectively disenfranchised.
It is extremely doubtful that PR could bring black-nationalist or white-supremacist parties to power in the House. Since blacks constitute only 12 percent of the population, and an even smaller proportion of the electorate, most black voters would have to support an extremist party for it to clear a five-percent threshold. It seems unlikely that they would, because more-moderate multiracial parties would more accurately reflect the economic concerns and values of most black voters. Even if a black-nationalist party passed the five-percent hurdle, it could be kept out of the House leadership. To elect one member to a five-member leadership committee, at least 17 percent of the House membership would be necessary. Black radicals would have to form coalitions with non-blacks.
What about white Klansmen and neo-Nazis? Right-wing nationalist parties have made gains in recent European elections, but these are protest votes by moderate voters who seek to change particular policies of centrist parties, not to replace those parties permanently with extremists. In the United States the support for Buchanan’s extreme conservatism, even in the Republican Party, appears to have been in the single digits, and support for David Duke-style racism vanishingly small. This suggests that the only viable far-right parties in a multiparty House would be Pat Robertson-style Christian conservatives, not Aryan Nation skinheads.
Those who claim that PR was responsible for the electoral successes of the National Socialist party in Weimar Germany are mistaken, according to no less an authority than Hermann Goering. Enid Lakeman, a British advocate of PR, writes of Weimar Germany that “if the British electoral system had been in use [the Nazi party] might have been grossly over-represented…Goering said at his trial that under the British system that  election would have given the Nazis every seat in the country, and he cannot have been far wrong.” Hitler would have loved the Anglo-American electoral system.