From the New York Times blog:
Among all the events that touched off the [Civil War]... — there is one that has been strangely ignored by most historians. True, it was less dramatic than the others. It occurred when enumerators traveled from door to door throughout America, counting up Easterners and Westerners; Northerners and Southerners; blacks and whites; freemen and slaves. The numbers that they came up with helped to split apart the Union.
Eighteen-sixty was a federal Census year, and the results had begun coming in early that autumn — with exquisitely poor timing, as far as Southern paranoia and Northern hubris were concerned. At the very moment that the slave states faced the imminent election of a Republican, antislavery president, a candidate who would win without a single vote in the Deep South, came other, equally shocking signs of change.