But as a Federal matter, slavery was not illegal. The Constitution itself said that escaped slaves must be returned to their masters. So how does the Fed Gov make a law against something the opposition could argue is a guaranteed right?
How can Federal law for the Territories be different from Federal law for the States?
There was little chance of territories voting for slavery. Whites had been fleeing the slave states for the free states for years, because they did not want to compete with slave labor.
And this is a point that I wish to elaborate on further. In history I was taught that the Northern people opposed slavery, and we were left to believe that they opposed it because they regarded it as immoral. Now while this was certainly true of some, it misrepresented the reasons why most Northern people opposed slavery.
What I discovered to be the truth is that the Northerners hated the black people even worse than did the Southerners. They passed laws that would prohibit them from settling in their states, Illinois being but one example. They didn't want black people near them, and they did not care at all if they were suffering or forced to labor. What they cared about is the possibility that the free labor represented by slavery would undermine their own labor and wages.
They hated slavery because slaves were "scabs" to their labor market. Because slaves worked for nothing, free men couldn't earn a decent income in a slave dominated market.
But we have been misled by historians into believing that most of the Northern people's objections to slavery were based on morality, instead of economics. No, it was pretty much the economics of it they couldn't stand.
The same areas are heavily unionized today, because those sections of the Nation are very protectionist regarding their labor and wages, and they were also that way in 1860.
A good place to start is A Patriots History of the US by Freeper Larry Schweikart.