How did western expansion create an imbalance in the US congress?

Western expansion in the United States led to an imbalance in Congress as new territories and states were added, often as slave or free states, which affected the delicate balance of power between pro-slavery and anti-slavery factions. The Missouri Compromise of 1820 and the Compromise of 1850 were attempts to address this imbalance by regulating the admission of states, but they ultimately highlighted the growing sectional tensions. As more western states were admitted, the struggle for representation intensified, contributing to the polarization that would culminate in the Civil War.