The correct answer is unpopular.
The Dred Scott decision was a Supreme Court case that ruled that slaves are considered property, not people. Along with this, the Supreme Court ruled that slaves had no legal rights that white citizens are bound to respect. With this in mind, the ruling official allowed slave owners to bring their slaves anywhere in the US, even the states that already outlawed slavery in their constitution.
This was widely unpopular in the North, as slavery (in theory) could now ecist in their state.