Final answer:
Senator Stephen A. Douglas proposed popular sovereignty to decide the slavery issue in the Kansas-Nebraska territories, which led to heightened tensions and conflict known as 'Bleeding Kansas'.
Step-by-step explanation:
Stephen A. Douglas proposed that the question of slavery in the Kansas-Nebraska territory be decided by popular sovereignty. This meant that the people living in the territories would determine whether or not to allow slavery through their vote. The Kansas-Nebraska Act included this principle and effectively repealed the Missouri Compromise line of 36° 30', opening new territories to the possibility of slavery which was a point of significant controversy accelerating the country towards civil strife.
Douglas introduced a bill to organize the Kansas and Nebraska territories, which opened the territories to slavery contrary to the Missouri Compromise. The act also applied the principle of popular sovereignty, dictating that the people of these territories would decide for themselves whether to adopt slavery.