There were two factors about the Republican campaign in the election of 1860 which disturbed the Southerners so badly that Southern states subsequently seceded. First was the Republican-party platform for 1860.
Basically, the Northern capitalists wanted the U.S. government to tax (only) the South deeply, to finance the industrialization of the North, and the necessary transportation-net to support that. In those days, there was no income tax. The federal government received most of its revenue from tariffs (taxes) on imported goods. The Southern states imported from England most of the manufactured goods they used, thus paid most of the taxes to support the federal government. (The Northerners imported very little.)

Second, the Republican party--unlike any of the other big political-parties that had come along--was purely a regional (Northern) party, not a national party. if the Republicans somehow managed to gain control of Congress AND the White House, they would then be able to use the federal government to enact and enforce their party platform--and thus convert the prosperous Southern-states into the dirt-poor agricultural colonies of the Northern capitalists. And given the 19th-century trends in demographics, the Southern states would never be able to reverse that process. The intent of the Declaration of Independence and the U.S. Constitution would then have been subverted completely: the Southern states would no longer be governed with the consent of the governed--but instead bullied mercilessly by the Northern majority. Why, then, remain in the Union?

This move took the Northern capitalists completely by surprise. The South was like the little boy who was forever crying "wolf." Southern states had been threatening to secede ever since the Tariff of Abominations and the days of Calhoun; the North no longer took those threats seriously. But with the South now gone, there would be no federal funding to industrialize the North--because the Northern citizenry would certainly never agree to be taxed to pay for it. And far worse than that, the many, many Northern-capitalists who had been earning fortunes factoring the Southern cotton-crop, transporting the cotton, and buying the cotton for New England textile-mills now faced financial ruin. The South normally bought its manufactured goods from Britain, anyway. Now, as a sovereign nation, the South could easily cut far better deals with the British financiers, shipowners, and textile mills to supply the South with all of the necessary support-services--leaving the Northern capitalists out in the cold.

The effects of the New England slave trade were momentous. It was one of the foundations of New England's economic structure; it created a wealthy class of slave-trading merchants, while the profits derived from this commerce stimulated cultural development and philanthropy. --Lorenzo Johnston Greene, “The Negro in Colonial New England, 1620-1776,” p.319.

Whether it was officially encouraged, as in New York and New Jersey, or not, as in Pennsylvania, the slave trade flourished in colonial Northern ports. But New England was by far the leading slave merchant of the American colonies.