I may have misremembered the Personal Liberty Laws of northern states as specifically being a big issue at the convention, although they were an enormous irritant to southerners, even though they had little actual effect. To be fair, these laws were entirely unconstitutional. (Which does not speak well for the Constitution.)
But I was entirely right about a federal slave code for the territories being demanded at the convention. When the demand was voted down, the southern delegates walked out of the Charleston convention.
Democrats tried again in Baltimore, and this time the southerners, and some of the northern delegates, walked out over the issue of whether all the delegates who had bolted from Charleston should be reseated or not. The seceding delegates then nominated Breckinridge for President.
http://en.wikipedia.org/wiki/1860_Democratic_National_Convention
The issue is covered well in Battle Cry of Freedom, which I just finished reading. The 3-way splintering of the Democratic Party pretty much ensured Lincoln’s election, which was probably the motive for some of the southern delegates behaving as they did. They wanted secession, and ensuring Lincoln’s election was the way to get it.
Personal liberty laws were passed in northern states in response to slave catchers pretending authority, providing false testimony, and forging documents, and under cover of that pretended authority, kidnapping free blacks from northern states.
Personal liberty laws forced the slave catcher to submit his evidencs to the state officials, and to give black residents of a free state the protection of the law. Since state officials often spotted the slave catchers’ forgeries, spotted inconsitencies in their testimony, and held that others testimony was more reliable, the slave power felt that the laws of civil procedure in free states (which is what personal liberty laws were) infringed on their rights to run the other states.
They were overrulled by the Taney court, in the following way: slave catchers could not demand state officials support in kidnapping alleged runaway slaves, but they could demand support from federal marshals. That issue had been settled long before the rebellion.
Dred Scott also ruled in favor of the slave power: Even when a man was not a runaway, and was brought to a free state, he still, held Taney, had no cause or standing to imagine that the laws of a free state held sway over anyone the slave power wanted to keep from freedom.
And still the slave power complained.