A large majority of Americans, both North and South, strongly rejected radical abolitionism. How, then, was radical abolitionism able to transform the public atmosphere regarding slavery, creating firece sectional polarization around the issue by the 1850's?

Answer: The answer is:

Because the abolitionist outcry had made a deep dent in the northern mind; many citizens had come to see the South as the land of the unfree and the home of a hateful institution; few northerners were prepared to abolish slavery outright, but a growing number, including Lincoln, opposed extending it to the western territories

Explanation:

In 1850 only 1,733 families owned more than 100 slaves each, and this select group provided the cream of the political and social leadership of the section and nation.

Much of the agitation in the North against the spread of slavery into the new territories in the 1840s and 1850s grew out or race prejudice, not humanitarianism.