Section #1 - The Life of Abraham Lincoln
Chapter 39: “Popular Sovereignty” Becomes Douglas’s Path to Assuaging the South

Douglas recognizes that his dual ambitions to drive western expansion through his hometown in Chicago, and to become President, depend on retaining support across the South.
He also knows that the Wilmot Proviso and the Free Soil Party reflect Northern opposition to expanding slavery.
“Pop Sov” is his lifeline to the South and it is deceptively plausible: “Let the settlers in each western territory hold elections to decide their own Slave State vs. Free State designation.”
What could make more sense than that in a democracy?
While the most cynical Southerners remain suspicious, they go along with it for the time being.