The south said they were going to secede if Abe got elected president, this has to do with the Dred Scott case because the north was against slavery and this case said that slaves and free blacks were not citizens and they had no right to sue in the first place
which I think is wrong, anybody can sue. But hey whatever my opinion
The legislative branch is the most powerful branch in government. They have the power to override a president's decision, stop laws from being passed, and basically control all decisions the governments makes