Concept
American electoral politics have been dominated by successive pairs of major political parties since shortly after the founding of the republic of the United States. Since the 1850s, the two largest political parties have been the Democratic Party and the Republican Party—which together have won every United States presidential election since 1852 and controlled the United States Congress since at least 1856. Despite keeping the same names, the two parties have evolved in terms of ideologies, positions, and support bases over their long lifespans, in response to social, cultural, and economic developmentsthe Democratic Party being the left-of-center party since the time of the New Deal, and the Republican Party now being the right-of-center party. Political parties are not mentioned in the U.S. Constitution (which predates the party system). The two-party system is based on laws, party rules, and custom. Several third parties also operate in the U.S. and, from time to time, elect someone to local office. Some of the larger ones include the Constitution, Green, Alliance, and Libertarian parties, with the latter being the largest third party since the 1980s. A small number of members of the US Congress, a larger number of political candidates, and a good many voters (35-45%) have no party affiliation. However, most self-described independents consistently support one of the two major parties when it comes time to vote, and members of Congress with no political party affiliation caucus meet to pursue common legislative objectives with either the Democrats or Republicans. The need to win popular support in a republic led to the American invention of voter-based political parties in the 1790s. Americans were especially innovative in devising new campaign techniques that linked public opinion with public policy through the party. Political scientists and historians have divided the development of America's two-party system into six or so eras or "party systems", starting with the Federalist Party, which supported the ratification of the Constitution, and the Democratic-Republican Party or the Anti-Administration party (Anti-Federalists), which opposed a powerful central government.