As far as I know, no one really spoke much about Slavery until afterward. The North disallowed any trading being done by the South at the time and taxed all of their imports and/or exports (I'm not sure I remember which or if it was just both). The South was much richer and the North knew that unless they siphoned some of that wealth away, the South would inevitably outgrow them. Obviously, the South reminded them of how they both got to this position - by dumping tea in the harbor... because of egregious taxation. It never had anything to do with slavery. That was merely an afterthought.