Economic Interests: The presence of slavery created competition in the labor market. Many Northern industries relied on wage labor and feared the competition from cheap slave labor. Northern workers worried that the expansion of slavery to the West would lead to competition from slave-produced goods, and thus lower wages for free labor.
Political power: The slave states held a disproportionate amount of power in the federal government due to the three-fifths compromise, which counted slaves as three-fifths of a person for the purpose of representation in the House of Representatives and the Electoral College. This gave the South an advantage in terms of political influence, and many Northerners resented the fact that their interests were not adequately represented.