How did many Southerners defend the institution of slavery?

Your answer would be that:

White Southerners defended the institution of slavery on a number of fronts. They said that it was necessary and they said that it was not forbidden, but they also argued that it was a positive good.

Southerners argued that slavery was an economic necessity. They argued that there was no way to get anyone to do the sort of labor that was needed for tobacco (and later cotton) cultivation without coercing them.