The Founding Fathers were wary of political parties, and didn't give them much thought when they wrote the Constitution. But almost before the ink was dry on the parchment, the two camps that had formed during the the Constitutional ratification contest -- the Federalists who supported the Constituion and the Anti-Federalists who opposed it -- were solidifying into political "factions," or parties.
George Washington famously warned against that trend in his 1796 Farewell Address. By then, though, the horse was already out of the barn. Thomas Jefferson and Alexander Hamilton -- two members of Washington's own Cabinet -- had chosen sides over the domestic and foreign issues of the day. Along with John Adams and James Madison, their political maneuvering in the 1790s layed the foundation for our two-party brand of politics.
We frequently refer to our two-party system and think in terms of Democrat and Republican, liberal and conservative, left and right. But our political differences aren't always so neatly contained, and third parties have repeatedly appeared on the political scene (see a map of the history of political parties here). That's especially true at times when voters believe that neither major party is addressing their concerns, as with the Republican Tea Party insurgency today.
From the 1850s through the 1940s, Congress typically included at least a few third-party representatives, from Free Soil in the 1850s to the American Labor Party in the 1930s and 1940s. But the influence of third parties registers not only on the few occasions that they win office independently. Third parties have served importantly as advocates of causes later embraced by the major parties and have contributed to their realignment. Today, Tea Party support has already swayed the outcome of elections throughout the primary season, including this week's victory of Christine O'Donnell in the Republican Senate primary in Delaware.