Congress
In 1852, Banks sought the Democratic nomination for a seat in the United States Congress. While it was at first granted, his refusal to disavow abolitionist positions meant support was withdrawn by party conservatives. He ended up winning a narrow victory anyway, with Free Soil support. In 1853, he presided over the state Constitutional Convention of 1853. This convention produced a series of proposals for constitutional reform, including a new constitution, all of which were rejected by voters. The failure, which was led by Whigs and conservative anti-abolitionist Democrats, spelled the end of the Democratic-Free Soil coalition.
In Congress, Banks sat on the Committee of Military Affairs. He bucked the Democratic party line by voting against the Kansas–Nebraska Act, which overturned the 1820 Missouri Compromise, using his parliamentary skills in an effort to keep the bill from coming to a vote. Supported by his constituents, he then publicly endorsed the abolitionist cause. His opposition came despite long stated support for Manifest Destiny (the idea that the United States was destined to rule the North American continent), which the bill's proponents claimed it furthered. In 1854, he formally joined the so-called Know Nothing cause, a secretive populist and anti-immigration nativist movement – officially named American Party since 1855. He was renominated for Congress by the Democrats and Free Soilers, and won an easy victory in that year's Know Nothing landslide victory. Banks was, along with Henry Wilson and Governor Henry J. Gardner, considered one of the political leaders of the Know Nothing movement, although none of the three supported its extreme anti-immigrant positions of many of its supporters.
In 1855, Banks agreed to chair the convention of a new Republican Party convention, whose platform was intended to bring together antislavery interests from the Democrats, Whigs, Free Soilers, and Know Nothings. When Know Nothing Governor Henry Gardner refused to join in the fusion, Banks carefully kept his options open, passively supporting the Republican effort but also avoiding criticism of Gardner in his speeches. Gardner was reelected. During the summer of 1855, Banks was invited to speak at an antislavery rally in Portland, Maine, his first major speaking opportunity outside Massachusetts. In the speech, Banks expressed his opinion that the Union did not necessarily need to be preserved, say that under certain conditions it would be appropriate to "let [the Union] slide". Future political opponents would repeatedly use these words against him, accusing him of "disunionism".
At the opening of the 34th U.S. Congress in December 1855, after the Democrats had lost their majority and only made up 35% of the House, representatives from several parties opposed to slavery's spread gradually united in supporting the Know Nothing Banks for Speaker of the U.S. House of Representatives. After the longest speakership contests on record, lasting from December 3, 1855, to February 2, 1856, Banks was chosen on the 133rd ballot, receiving 103 votes out of 214 cast, or five less than an absolute majority. The coalition supporting him was formed by his American Party (known as the Know Nothing Party) and the Opposition Party, which opposed the Democrats, marking the first form of a coalition in congressional history. This victory was lauded at the time as the "first Republican victory" and "first Northern victory" – although Banks is officially affiliated as Speaker from the American Party – and greatly raised Banks's national profile. He gave antislavery men important posts in Congress for the first time, and cooperated with investigations of both the Kansas conflict and the caning of Charles Sumner on the floor of the Senate. Because of his fairness in dealing with the numerous factions, as well as his parliamentary ability, Banks was lauded by others in the body, including former Speaker Howell Cobb, who called him "in all respects the best presiding officer [I] had ever seen."
Banks played a key role in 1856 in bringing forward John C. Frémont as a moderate Republican presidential nominee. Because of his success as speaker, Banks was considered a possible presidential contender, and his name was put in nomination by supporters (knowing that he supported Frémont) at the Know Nothing convention, held one week before the Republicans met. Banks then refused the Know Nothing nomination, which went instead to former President Millard Fillmore. Banks was active on the stump in support of Frémont, who lost the election to James Buchanan. Banks easily won reelection to his own seat, though Democrats regained control of the House of Representatives. He was not re-nominated for speaker when the 35th Congress convened in December 1857.
