In the 1850s, most white northerners were not abolitionists and yet there was widespread opposition in the North to the Kansas-Nebraska Act. Why?

The Kansas-Nebraska Act neutralized the Missouri Compromise. The Missouri compromise was responsible for slave-free accommodation.

Senator Stephen Douglas convinced Congress to permit the white males in an area to have the right to vote for slave or to be free from them. The people who voted in favor of slaves moved into areas that were free from slavery under the Missouri Compromise. The Settlers from the North settled in the Kansas Territory and were not interested in slave labor.

Although they were not abolitionists but they didn’t want any form of competition for land from slave holders who paid nothing to the slaves. All they wanted was a country with only whites and without black people or slavery.

i honesty don't know, good luck tho dawg