The American Party, known as the Native American Party before 1855^[a] and colloquially referred to as the Know Nothing, Know-Nothings, or the Know Nothing Party, was an Old Stock nativist political movement in the United States from the 1840s through the 1850s. Members of the movement were required to say "I know nothing" whenever they were asked about its specifics by outsiders, providing the group with its colloquial name.^[4]

Supporters of the Know Nothing movement believed that an alleged "Romanist" conspiracy to subvert civil and religious liberty in the U.S. was being hatched by Catholics. Therefore, they sought to politically organize native-born Protestants in defense of their traditional religious and political values. The Know Nothing movement is remembered for this theme because Protestants feared that Catholic priests and bishops would control a large bloc of voters. In most places, the ideology and influence of the Know Nothing movement lasted only one or two years before it disintegrated due to weak and inexperienced local leaders, a lack of publicly proclaimed national leaders, and a deep split over the issue of slavery. In parts of the South, the party did not emphasize anti-Catholicism as frequently as it emphasized it in the North and it stressed a neutral position on slavery,^[5] but it became the main alternative to the dominant Democratic Party.^[4]

The Know Nothings supplemented their xenophobic nativist views with populist appeals. At the state level, the party was, in some cases, progressive in its stances on "issues of labor rights and the need for more government spending"^[6] and furnished "support for an expansion of the rights of women, the regulation of industry, and support of measures which were designed to improve the status of working people".^{[7][attribution needed]} It was a forerunner of the temperance movement in the U.S.^[4]

The Know Nothing movement briefly emerged as a major political party in the form of the American Party.^[4] The collapse of the Whig Party after the passage of the Kansas–Nebraska Act left an opening for the emergence of a new major political party in opposition to the Democratic Party. The Know Nothing movement managed to elect congressman Nathaniel P. Banks of Massachusetts and several other individuals into office in the 1854 elections, and it subsequently coalesced into a new political party which was known as the American Party. Particularly in the South, the American Party served as a vehicle for politicians who opposed the Democrats. Many of the American Party's members and supporters also hoped that it would stake out a middle ground between the pro-slavery positions of Democratic politicians and the radical anti-slavery positions of the rapidly emerging Republican Party. The American Party nominated former president Millard Fillmore in the 1856 presidential election, but he kept quiet about his membership in it and he personally refrained from supporting the Know Nothing movement's activities and ideology. Fillmore received 21.5% of the popular vote in the 1856 presidential election, finishing behind the Democratic and Republican nominees.^[8] Henry Winter Davis, an active Know-Nothing, was elected on the American Party ticket to Congress from Maryland. He told Congress that "un-American" Irish Catholic immigrants were to blame for the recent election of Democrat James Buchanan as president, stating:^[9]