What is Douglas's purpose in the following paragraph?

Thus you see that up to 1853-54, the Whig party and the Democratic party both stood on the same platform with regard to the slavery question. That platform was the right of
the people of each State and each Territory to decide their local and domestic institutions for themselves, subject only to the federal constitution

A. To prove he was right

B. To show that Lincoln was wrong
hile some Northern ones go South, and become most cruel slave-masters.

C. To prove that America had always agreed about slavery

D. To support his point that slavery is a state issue, not a federal issue