There are several possible reasons. The first is that it increased the role of the federal government within state affairs, because the states had to defer to this federal intervention. The second is that this law required that all citizens support slavery, which was not always the case.
The Fugitive Slave Act of 1850 angered Northerners because it increased federal control over state laws, denied legal rights to accused fugitives, and required citizens to assist in capture under threat of penalties. Additionally, the act led to widespread corruption and the wrongful enslavement of free Blacks, intensifying opposition to slavery in the North. This law was pivotal in the growing conflict between Northern and Southern states regarding the institution of slavery.
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