Final answer:
Many northerners opposed the new Fugitive Slave Act primarily because of their opposition to slavery's expansion.
Step-by-step explanation:
Many northerners were opposed to the new Fugitive Slave Act created by the Compromise of 1850 primarily because of their opposition to slavery's expansion. The Fugitive Slave Act required northerners to assist in the arrest and return of fugitive slaves, even if they personally opposed slavery. The act also undermined local and state laws in the North, and some northerners resented being compelled to enforce a law that they believed was unjust.