This is all to say that in the United States, a successful third party isn’t necessarily one that wins national office. Instead, a successful third party is one that integrates itself or its program into one of the two major parties, either by forcing key issues onto the agenda or revealing the existence of a potent new electorate.
Take the Free Soil Party.
During the presidential election of 1848, after the annexation of Texas, the Mexican-American War and the Treaty of Guadalupe Hidalgo, a coalition of antislavery politicians from the Democratic, Liberty and Whig Parties formed the Free Soil Party to oppose the expansion of slavery into the new Western territories. At their national convention in Buffalo, the Free Soilers summed up their platform with the slogan “Free soil, free speech, free labor, free men!”
The Free Soil Party, notes the historian Frederick J. Blue in “The Free Soilers: Third Party Politics, 1848-1854,” “endorsed the Wilmot Proviso by declaring that Congress had no power to extend slavery and must in fact prohibit its extension, thus returning to the principle of the Northwest Ordinance of 1787.” It is the duty of the federal government, declared its platform, “to relieve itself from all responsibility for the existence of slavery wherever that government possesses constitutional power to legislate on that subject and is thus responsible for its existence.”
This was controversial, to put it mildly. The entire two-party system (the first being the roughly 30-year competition between the Federalists and the Jeffersonian Republicans) had been built to sidestep the conflict over the expansion of slavery. The Free Soil Party — which in an ironic twist nominated Martin Van Buren, the architect of that system, for president in the 1848 election — fought to put that conflict at the center of American politics.
It succeeded. In many respects, the emergence of the Free Soil Party marks the beginning of mass antislavery politics in the United States. It elected several members to Congress, helped fracture the Whig Party along sectional lines and pushed antislavery “Free” Democrats to abandon their party. The Free Soilers never elected a president, but in just a few short years they transformed American party politics. And when the Whig Party finally collapsed under the weight of its own contradictions, after General Winfield Scott’s defeat in the 1852 presidential election, the Free Soil Party would become, in 1854, the nucleus of the new Republican Party, which brought an even larger coalition of former Whigs and ex-Democrats together with Free Soil radicals under the umbrella of a sectional, antislavery party.
There are a few other examples of third-party success. The Populist Party failed to win high office after endorsing the Democratic nominee, William Jennings Bryan, for president in 1896 but went on to shape the next two decades of American political life. “In the wake of the defeat of the People’s Party, a wave of reform soon swept the country,” the historian Charles Postel writes in “The Populist Vision”: “Populism provided an impetus for this modernizing process, with many of their demands co-opted and refashioned by progressive Democrats and Republicans.”