No he couldn't. The North was not in rebellion.
He didn't because it was not a matter of principle as you Northerners like to claim, it was a matter of economics, for both the North and the South. And it was Lincoln's way of punishing the South.
Oh baloney. The North was a more powerful industrialized area as proven by the Civil War. Keeping down the South didn't prop up the North. South Carolina made it abundantly clear in their Declaration of Secession that Lincoln had an hostility to slavery.