The North
By 1830, all slavery had been abolished in the North. When they did have slavery, they felt it was morally wrong, and they should rely mostly on free labor.
The South
In the south they firmly believed in slavery, unlike the North. They felt they could not survive without slave labor, and the economy would crash if they abolished it.
The West
Westward expansion caled for slavery to expand into new territories some wanted slavery, and some didn't. The issue passed through the house, but not the senate.