Why was the popular sovereignty a solution and then a failure?

Popular sovereignty was an idea pushed on Douglas by Southern senators in order to pass his Kansas-Nebraska Act but Douglas made it a centerpiece of his policy because he saw it as the means to unite the Democrats, North and South, behind him.  Popular sovereignty had the opposite effect: it polarized political opinion even more.  Popular sovereignty in practice touched off something very close to a civil war in Kansas as pro-slavery and anti-slavery factions organized their own state constitutional conventions and led to wave of bushwhacking and political murders.

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pittsburgh, Pennsylvania