Why did many Northerners oppose the expansion of slavery into the west?

While the South utilized slavery to sustain its culture and grow cotton on plantations, the North prospered during the Industrial Revolution. Northern cities, the center of industry in the United States, became major metropolises due to an influx of immigrants. With this willing and cheap workforce, the North did not require a slave system. Although some northerners found the institution of slavery morally reprehensible, most did not believe in complete racial equality either. Slavery became even more divisive when it threatened to expand westward because non-slave holding white settlers did not want to compete with slaveholders in the new territories.