The major issue that led to the split of the Democratic and Republican parties in the 19th century was slavery. As the United States expanded westward, debates over whether new territories would allow slavery intensified. The Democratic Party, particularly in the South, largely supported slavery, while the newly formed Republican Party in the 1850s emerged largely in opposition to its expansion. The 1860 election of Abraham Lincoln, a Republican, further polarized the political landscape, leading to the secession of Southern states and eventually fueling the outbreak of the Civil War.