How did settlement of the west increase tension between the North and the south?

The opening of the western territories came at a time when the North and South were at odds over the question of slavery. Southerners wanted to add to their number, political and economic power by hopefully allowing slavery in the territories or at least letting the people there choose for themselves. Northerners did not want to add more slave states. They viewed slavery an abhorrent evil as well as a system that devalued the work of the free working man. They also did not want to increase the voting power of the South.