How did westward expansion contribute to sectional tensions in the united states?

This isn't a full answer, but the southern states and the northen states needed the new states to be on their sides (mostly pro/anti slave laws) bc if too many of the new states went to one side, the other side would be out-voted in Washington and every state would have to be pro/anti slave even if the state in question didn't agree. 

Pls note I'm assumeing that by sectional tenstion the question is talking about
South/North conflict leading up to the Civil War.