How did many southerners defend the institution of slavery?

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. 

in this part of the declaration it refers to the point when american people didn't recognize the british domain and   breaks the political bands with britain, they forced a respectful separation.

mankind was created and entitled equally with rights   which were life, freedom and the seeking for happiness.

united states government;