Tracing their roots to Thomas Jefferson's Democratic-Republicans, the Democrats did not fully coalesce as a party until Andrew Jackson's presidency, when they rallied around the administration against increasing opposition from the nascent Whig Party. Democrats generally supported a weak federal government and limited budget, with a strong belief in states' rights. They envisioned America as an agricultural, rather than industrial, republic and resisted any perceived assault on personal liberty. These principles enabled the party to maintain a national membership through the 1830s and 1840s, but occasional splits did occur. In the 1830s and 1840s a group of Democrats popularly known as the "locofocos" broke with their party by favoring libertarian economic policies and the breakup of monopolies. However, a more serious challenge to party unity emerged at mid-century, as increasing sectional tensions over slavery and westward expansion resulting from the Democrat-favored Mexican War increasingly tied the party to southern slaveholding interests.
Stephen A. Douglas emerged as the party's de facto leader during the first half of the 1850s. His promotion of popular sovereignty as the best means of deciding if new states would be free or slave appealed to the party's slaveholding elite and broader adherence to states' rights. However, fissures began to emerge as the decade progressed. The Kansas-Nebraska split the party between Douglas Democrats and the "Free Soil" wing of the party that opposed the expansion of slavery. President James Buchanan further split the party in 1857 over slavery in Kansas. Douglas broke with Buchanan over the Lecompton Constitution, leading the Buchanan to foster a separate National Democratic party in Illinois. The so-called "Danites," named for a secret order of Mormons, ran their own candidates for the state legislature and openly supported Abraham Lincoln in the 1858 Senate campaign. Tension between these various factions exploded in the Democratic National Convention of 1860. Douglas went into the convention as the party's favored presidential candidate but without the two-thirds majority required to secure his nomination. In exchange for their support, southern delegates demanded he adopt a platform guaranteeing the rights of slaveholders in the territories, but northern Democrats would not acquiesce. After a series of subsequent conventions and debates, southern delegates nominated their own candidate, John C. Breckinridge, on a firmly pro-slavery platform while northern Democrats nominated Douglas. This geographic division allowed Republican candidate Abraham Lincoln to carry the electorate and assume the presidency in 1861.
Following the secession crisis and the outbreak of the Civil War, the Democratic Party effectively split in two, as southern Democrats controlled the Confederacy as a one-party state and northern Democrats operated as a loyal opposition to the Lincoln administration and a Republican-controlled Congress. Questions over the scope of that opposition prevented northern Democrats from adopting a consistent platform, as some supported the war but not emancipation while others sought an immediate end to hostilities. The latter position allowed Republicans to paint Democrats as disloyal traitors, often using the pejorative term "Copperheads." This conflict was most visible during the presidential election of 1864, in which Democrat candidate George B. McClellan failed to fully overcome the party's negative image. Lincoln defeated McClellan and the Republicans maintained their majorities in Congress throughout the rest of the war.
Robert Allen Rutland, The Democrats: From Jefferson to Clinton (Columbia: University of Missouri Press, 1995), 54-132; David M. Potter and Don E. Fehrenbacher, The Impending Crisis, 1848-1861 (New York: Harper & Row, 1976), 63-554; David Donald, Lincoln(New York: Simon & Schuster Paperbacks, 1995), 196-539.