Before the civil war began, which issue played the greatest role in increasing tension and conflict between northern and southern states?

Before the beginning of the Civil War, the biggest issue was slavery.

In particular, the North and South fought over how to admit new States into the Union and whether the vile institution of slavery should be permitted.

Slavery

Explanation:

The institution of slavery was the largest factor in helping to spark the Civil War. The institution of slavery and its existence has caused significant problems between Northerners and Southerners, especially as the United States grew. Fighting over whether or not new territories/states would have slavery caused a serious of failed compromises like the Missouri Compromise, the Compromise of 1850, and the Kansas-Nebraska Act. This continually fighting over slavery and its existence was what sparked the state of South Carolina to secede from the Union after the election of president Abraham Lincoln.