The United States has had five or six party systems, pairs of dominant parties, each with distinctive constituencies and positions. There have been numerous smaller parties over the US's history, but most of them have either remained insignificant or else have joined one of the major parties.
The United States's political system has long had antiquated features like first-past-the-post voting. Duverger's Law then sets in, guaranteeing that the US always has at most two major parties. However, that has not kept the parties from realigning every now and then. These realigning usually take place in periods of Liberalism and reform in Arthur Schlesinger's US history cycles, or shortly before.
First Party System: 1796 - 1824Edit
The creators of the US Constitution wanted no political parties, but they could not keep people from dividing themselves into factions.
The Feds dominated until about 1800, then the D-R's after that, with the Feds gradually dwindling to a few isolated strongholds, and then disappearing.
Second Party System: 1828 - 1854Edit
The Democratic-Republican Party split in two.
Third Party System: 1854 - 1896Edit
Whig Party collapsed, Republican Party emerges
- Democratic Party: southern, associated with liturgical churches (Roman Catholic, some Protestant)
- Republican Party: northern, associated with pietistic churches (Protestant)
Fourth Party System: 1896 - 1932Edit
Republican Party dominates, both parties continue
- Democratic Party: "wet", anti-Prohibition
- Republican Party: "dry", pro-Prohibition
Fifth Party System: 1932 - 1968 Edit
The New Deal Coalition
Both parties continue, though despite that, more parties continue to be created. Of course, it makes more sense to reform the Democratic Party to be more liberal so that more liberals will represent it. After all, Democrats shouldn't try to mimic the GOP.
However, historians of US political history argue about whether it still continues, or whether it has ended and a Sixth Party System has emerged. Part of the problem is that the transition has been very gradual, and not as fast as the previous transitions have been, with the previously-dominant party losing big in the House twice in a row.
It does seem like there has been a major realignment over the last half century. Southern Democrats have moved to the Republican Party, while Northern states have become more and more Democratic. This realignment started back when the Democratic Party started to support the Black Civil-rights movement, and it has continued ever since, most recently with the resignation of Maine Republican Senator Olympia Snowe.
As the Republican Party goes farther in changing from the party of Abraham Lincoln to the party of Jefferson Davis, it misses an opportunity to get additional voters: Hispanics. The Republicans could appeal to them with social Conservatism and anti-welfarism, but instead it prefers to associate itself with those who suspect Hispanics of mostly being Illegal immigrants. This increases the risk that the Republicans could go the way of the Federalists and the Whigs: gradually dwindling into irrelevance.