<P> Political scientists and historians have divided the development of America's two - party system into five eras . The first two - party system consisted of the Federalist Party, who supported the ratification of the Constitution, and the Democratic - Republican Party or the Anti-Administration party (Anti-Federalists), who opposed the powerful central government, among others, that the Constitution established when it took effect in 1789 . </P> <P> The modern two - party system consists of the Democratic Party and the Republican Party . Several third parties also operate in the U.S., and from time to time elect someone to local office . The largest third party since the 1980s is the Libertarian Party . </P> <P> Besides the Constitution, Green, and Libertarian parties, there are many other political parties that receive only minimal support and only appear on the ballot in one or a few states . </P> <P> Some political candidates, and many voters, choose not to identify with a particular political party . In some states, independents are not allowed to vote in primary elections, but in others, they can vote in any primary election of their choice . Although the term "independent" often is used as a synonym for "moderate," "centrist," or "swing voter," to refer to a politician or voter who holds views that incorporate facets of both liberal and conservative ideologies, an independent can be of any ideological or political persuasion . </P>

What are the two periods that changed the political landscape in america