In the Northern states slavery was forced to compete with free white labor in a way which was not true of the plantation economy of the South.

White workers continually complained that slavery was keeping their wages down and unemployment up, and in 1737 the governor of New York had asked the Legislature to investigate the charges that slave competition contributed to unemployment.

While this attack had helped to undermine slavery, it had also exacerbated tension between black and white labor.

The continual flow of runaways from the South brought an increasing supply of cheap black labor to compete with white workers, and the friction between the two races continued.

While many of the runaways, like Frederick Douglass, had worked as skilled craftsmen in the South, they found economic discrimination in the North limiting them to menial labor.
After 1830, when the tide of European immigration began to swell, the competition for jobs grew even sharper, and blacks found that even menial jobs were being taken over by the new European immigrants.