Stephen Arnold Douglas (né Douglass; April 23, 1813 – June 3, 1861) was an American politician and lawyer from Illinois. As a U.S. senator, he was one of two nominees of the badly split Democratic Party to run for president in the 1860 presidential election, which was won by Republican candidate Abraham Lincoln. Douglas had previously defeated Lincoln in the 1858 United States Senate election in Illinois, known for the pivotal Lincoln–Douglas debates. Earlier, Douglas was one of the brokers of the Compromise of 1850, which sought to avert a sectional crisis; to further deal with the volatile issue of extending slavery into the territories, Douglas became the foremost advocate of popular sovereignty, which held that each territory should be allowed to determine whether to permit slavery within its borders. This attempt to address the issue was rejected by both pro-slavery and anti-slavery advocates. Standing 5 feet 4 inches (1.63 m) tall, Douglas was nicknamed the "Little Giant" because he was short in physical stature but a forceful and dominant figure in politics.

Born in Brandon, Vermont, Douglas migrated to Jacksonville, Illinois, in 1833 to establish a legal practice. He experienced early success in politics as a member of the newly formed Democratic Party, serving in the Illinois House of Representatives and various other positions. He was appointed to the Supreme Court of Illinois in 1841. In 1843, he was elected to the U.S. House of Representatives and therefore resigned from the Supreme Court of Illinois. Douglas became an ally of President James K. Polk and favored the annexation of Texas and the Mexican–American War. He was one of four Northern Democrats in the House to vote against the Wilmot Proviso, which would have banned slavery in any territory acquired from Mexico.

The Illinois legislature elected Douglas to the U.S. Senate in 1847, and Douglas emerged as a national party leader during the 1850s. Along with Senator Henry Clay of the Whig Party, he led the effort to pass the Compromise of 1850, which settled some of the territorial issues arising from the Mexican–American War. Douglas was a candidate for president at the 1852 Democratic National Convention but lost the nomination to Franklin Pierce. Seeking to open the west for expansion, Douglas introduced the Kansas–Nebraska Act in 1854. Though Douglas had hoped the Kansas–Nebraska Act would ease sectional tensions, it elicited a strong reaction in the North and helped fuel the rise of the anti-slavery Republican Party. Douglas once again sought the presidency in 1856, but the 1856 Democratic National Convention instead nominated James Buchanan, who went on to win the election. Buchanan and Douglas split over the admission of Kansas as a slave state, and Douglas successfully helped block the admission, accusing a pro-slavery Kansas legislature of having conducted an illegitimate and unfair election. Kansas eventually came into the Union as a free state.

During the Lincoln–Douglas debates, Douglas articulated the Freeport Doctrine, which held that territories could effectively exclude slavery despite the Supreme Court's ruling in the 1857 case of Dred Scott v. Sandford. Disagreements over slavery led to the bolt of Southern delegates at the 1860 Democratic National Convention. The rump convention of Northern delegates nominated Douglas for president, while Southern Democrats threw their support behind John C. Breckinridge. In the 1860 election, Lincoln and Douglas were the main candidates in the North, while most Southerners supported either Breckinridge or John Bell of the Constitutional Union Party. Campaigning throughout the country during the election, Douglas warned of the dangers of secession and urged his audiences to stay loyal to the United States. Ultimately, Lincoln's strong support in the North led to his victory in the election. After the Battle of Fort Sumter, Douglas rallied support for the Union, but he died in June 1861.

He was born Stephen Arnold Douglass in Brandon, Vermont, on April 23, 1813, to physician Stephen Arnold Douglass and his wife, Sarah Fisk. The younger Douglas would drop the second "s" from his name in 1846, the year after the publication of Frederick Douglass's first autobiography; it is unknown if these two events were connected. Douglas's paternal ancestors had migrated to New England in the 17th century, and his paternal grandfather, Benajah Douglass, served several terms in the Vermont House of Representatives. Douglas's father died when Douglas was just two months old. Douglas, his mother, and his older sister moved to the farm that his mother and her bachelor brother, Edward Fisk, had inherited from their father. Douglas received an elementary education at the local school in Brandon. As a teenager, Stephen left the family farm for Middlebury, Vermont, and apprenticed himself to a cabinetmaker named Nahum Parker. He began reading political literature and engaging in discussions with his employer and other young men. Douglas came to have great admiration for Andrew Jackson. He left Middlebury and returned to Brandon after he had grown dissatisfied with his employer. He began another apprenticeship with another cabinetmaker, Deacon Caleb Knowlton, but also quit this employer after less than a year.

Douglas moved back in with his mother and decided to enroll as a student at Brandon Academy in order to pursue a professional career. Soon, however, his sister married a man from western New York. Stephen's mother later married this man's father, Gehazi Granger. The whole family then relocated to the Granger estate in New York, Stephen included. He was 17 years old at that time, and soon continued his education at nearby Canandaigua Academy. He began the study of Latin and Greek and showed particular skill as a debater. At this point, he already may have been looking forward to a career as a politician. At Canandaigua Academy, Douglas frequently gave speeches supporting Andrew Jackson and Jackson's Democratic Party. A prominent local attorney, Levi Hubbell, allowed Douglas to study under him and, while a student in Hubbell's office, Douglas became friendly with Henry B. Payne, who was studying law at the nearby office of John C. Spencer.

In 1833, aged just 20, Douglas decided to leave New York and move West. Despite his mother's protests and the fact that he had not yet completed his studies at the academy, Stephen ventured out on his own. The newer states of the West had easier conditions for admission to the bar and he was eager to begin his professional career. And so, with his purposes only partially formed and only enough money for immediate needs, he began his westerly drift. After a short stay in Buffalo, New York, and a visit to Niagara Falls, Douglas took a steamboat to Cleveland, Ohio. He initially had hoped to establish himself there, knowing that it would only take him a year to gain admission to the bar in Ohio as opposed to four years in Vermont. Within a few days, however, he was stricken with malarial typhoid and was very ill for four months. He very easily could have died. After paying all of his bills, he still had forty dollars left. Douglas decided to push farther west.

He took a canal boat from Cleveland to the southern Ohio town of Portsmouth, then went west to Cincinnati. Douglas still had no well-defined purpose and drifted from city to city, stopping in Louisville and St. Louis. His money now almost all spent, he had to find work soon. Finding no luck in St. Louis, he became convinced that he must find some small country town. Upon hearing that Jacksonville in Illinois was a thriving settlement, he decided to try his luck there. In Jacksonville, Douglas befriended attorney Murray McConnel, a friendship that would continue throughout Douglas's life. McConnel, having no employment to offer Douglas, advised him to go to the town of Pekin, Illinois, and open a law office there, believing Pekin was destined to become a major shipping and marketing hub. With books gifted to him by McConnel, Douglas waited in the town of Meredosia for a steamboat that would take him to Pekin via the Illinois River. Douglas waited a week until learning that the only boat expected on the river at that time of year had blown up. Broke and in desperate need of employment, Douglas rode with a farmer to the village of Exeter to open a school. The townspeople informed Douglas that a school could probably be opened in Winchester, ten miles away; a distance that Douglas traveled on foot. After acquiring enough money and a license to practice law, Douglas moved back to Jacksonville. Morgan County was then only sparsely populated and still very much 'wild country'. The open prairie lands were a revelation to Douglas. Having grown up in the hills of Vermont, he found these lands to be like nothing he had previously seen. Years later he said, "I found my mind liberalized and my opinions enlarged when I got on these broad prairies, with only the heavens to bound my vision, instead of having them circumscribed by the little ridges that surrounded the valley where I was born." Douglas settled in Jacksonville in November 1833.