In the 2nd Lincoln-Douglas debate held at Freeport, Lincoln asked Douglas: could the settlers of a territory vote to exclude slavery before it became a state? Lincoln implied that popular sovereignty was an empty term because the Dred Scott decision said territories could not exclude slavery.
Douglas’s response to this question with the Freeport Doctrine. He said that slavery couldn’t exist anywhere without support of local police regulations so the people of a territory were Free-Soilers, then all they had to do was elect representatives who wouldn’t enforce slave property laws. Basically, people could get around the Dred Scott decision.