Announced by the Supreme Court in March 1857, a few days after James Buchanan's inauguration, the Dred Scott Decision marked an attempt by southern and Democratic justices on the Court to provide a proslavery, prosouthern resolution to disputes over slavery expansion. The case originated with a Missouri slave named Dred Scott who in the 1830s had lived with his owner in the free state of Illinois and then in Wisconsin Territory, from which slavery had been prohibited by the Missouri Compromise's ban on slavery north of the 36° 30' line. In the late 1840s, having returned once again to Missouri, Scott sued for his freedom in Missouri state courts on the basis that once having lived in free territory, he was free. When this plea was rejected by Missouri judges who ruled that Scott remained a slave, Scott sued his new owner, a New York citizen named John A. Sanford, in federal courts under the Constitution's diverse citizenship clause, and that case eventually came before the Supreme Court on appeal from a lower federal court that upheld the original decision in Missouri that Scott remained a slave.

Led by Chief Justice Roger B. Taney, the Court's majority, all of whom were Democrats and five of whom were Southerners, ruled as follows: 1) that Scott was still a slave and thus had no standing to sue in federal courts; 2) that even if he were free, Scott would still lack standing since free blacks were not citizens, indeed that blacks, in Taney's infamous phrase, "had no rights which the white man was bound to respect"; and 3) that the Missouri Compromise prohibition of slavery on which Scott rested his case was itself unconstitutional since it deprived slaveholders of their property without due process in violation of the Fifth Amendment.

In short, the southern and Democratic majority of the Court ruled that the central platform plank of the Republican party — the demand for congressional prohibition of slavery from all federal territories — was unconstitutional. While the decision made no allusion to the power of a territorial legislature to bar slavery, it immediately raised questions as well about the Democratic formula of popular sovereignty since men, and especially Southerners, immediately asked how Congress could delegate to a territorial legislature a power to prohibit slavery that it itself did not constitutionally possess. Stephen A. Douglas, Democrats' chief advocate of popular soveignty was especially embarrassed by this question. Thus the Dred Scott decision laid the groundwork for much of the jousting between Douglas and Lincoln during their famous 1858 debates.