Why did some Northerners in Congress disapprove of the Kansas-Nebraska Act?

The Kansas-Nebraska Act used popular sovereignty, allowing settlers in Kansas and Nebraska to decide whether to permit slavery. This overturned the Missouri Compromise’s restriction on slavery north of the 36°30′ line, so slavery could spread into territories where it had previously been prohibited. That is why some Northerners opposed it; it did not force existing Northern states to accept slavery.

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