Final answer:
Senator Stephen Douglas's organization of territories west of Missouri and Iowa was driven by the goal to unify the Democratic Party and appeal to the South by repealing the Missouri Compromise line and enabling popular sovereignty. He also aimed to route a transcontinental railroad through Chicago for economic growth and political gain.
Step-by-step explanation:
Senator Stephen Douglas was motivated to organize land west of Missouri and Iowa into territories primarily by his desire to promote the Democratic Party's unity after sectional conflicts over slavery and patronage jobs. This led him to introduce the Kansas-Nebraska bill in 1854. The act not only aimed at reconciling party divisions but also appealed to southern colleagues by proposing the repeal of the Missouri Compromise line and introducing the principle of popular sovereignty to allow territories to decide on the issue of slavery themselves. An important objective for Douglas was to route the eastern terminus of a transcontinental railroad through Chicago to stimulate economic growth and potentially boost his presidential ambitions.