"the sectional conflict between northern and southern states that eventually led to the civil war can be traced as far back as the constitutional convention of 1787." which fact provides the best support for this statement?

The issue of how many representatives in the house was debated heavily in the Constitutional Conventions. Eventually they decided that the population the whites plus three-fifths of their slaves. This shows there was already tension about the way the government would be run and the distribution of power between the states.