What doctrine, stated by Stephen Douglas, claimed that territories could limit slavery through popular sovereignty?

The doctrine referred to is indeed the Freeport Doctrine, articulated by Stephen Douglas during a debate with Abraham Lincoln in 1858. This concept proposed that while the U.S. Supreme Court had ruled in the Dred Scott decision that Congress could not prohibit slavery in the territories, settlers in those territories could effectively limit or exclude slavery by not establishing the necessary local laws to protect it. This idea of "popular sovereignty" emphasized that the people living in a territory should have the right to decide whether to permit slavery, rather than it being dictated by federal law or ruling.

The Freeport Doctrine was significant because it highlighted the tensions between state rights and federal authority regarding slavery, and it reflected the divisions within the Democratic Party over this issue. It also illustrated Douglas's attempt to reconcile the interests of pro-slavery and anti-slavery factions while seeking support for his ongoing political career.