Final answer:
Northerners were concerned about the impact of expanding the number of states on the balance of power in Congress.
Step-by-step explanation:
Northerners were concerned that adding more states would shift the balance of power in Congress. They feared that if new slave states were added, it would give more power to the Southern states and threaten the anti-slavery movement. They were also concerned that adding new states would dilute their influence and representation in Congress.
Learn more about Northerners' concerns about expanding the number of states