Sectionalism is a tendency among sections in bureaucracy to blindly focus on the interest of a section at the expense of the whole. In national politics, sectionalism is often a precursor to separatism. In a political context, sectionalism is loyalty to the interests of one's own region or section of the country, rather than the nation as a whole.
In the United States, sectionalism became a major problem and the nation's fastest growing influence when Missouri applied for statehood in 1819. Most of those living in Missouri wanted slavery to be allowed in the state. However, if this had happened, there would be twelve states that allowed slaves and only eleven that did not, consequently upsetting the balance of power in the United States Senate. The Missouri Compromise solved the problem and restored balance in the United States Senate by adding two new states to the Union, Maine and Missouri. Maine would be a state where slavery was illegal and Missouri would have slavery.
From the early 1800s up to the American Civil War in the 1860s, there was the Nullification Crisis where advocates of sectionalism (mostly in the South) led by former Vice President and South Carolina senator John C. Calhoun believed that state legislatures could deny enforcement of Federal law if they felt it violated the United States Constitution. Many feared that if individual states could "pick and choose" federal laws, it would be the end of the Union.
Sectionalism played a minor part in President Abraham Lincoln's winning of the election in 1860. Because Lincoln did not support slavery, and all of the Northern States (with most of the popular votes) also did not support slavery, Lincoln won more of the popular votes in those states and thus won their votes in the United States Electoral College. Lincoln's win caused many Southern states to suspect that their rights would not be protected so they seceded from the Union and established the Confederate States of America, which eventually led to the American Civil War.
The Solid South is an example of Sectionalism. From 1880-1924,the electoral system of the entire South voted exclusively for the Democratic Party.