The abolitionist movement and the underground railroad increased tensions between the North and South because they highlighted the problem of slavery while fighting against it directly. Southern states felt attacked by these movements.
Explanation:
Slavery was an important feature of Souther identity. Because of their strong defense of slavery they felt personally attacked by any critics of the slavery system.
Thus as the abolitionist movement and the underground railroad gained momentum these questions got more intense. In the South the hate for abolitionism got to the point where censorship was overt: abolitionism was illegal and president Andrew Jackson prohibited the postal service from delivering abolitionist publications to the South.
These tensions were not resolved and would lead to the Civil War.
