Final answer:
The attempt to extend the Missouri Compromise line westward to the Pacific Ocean was repeatedly rejected by the House, leading to the Kansas-Nebraska Act which repealed the Missouri Compromise line.
Step-by-step explanation:
The unsuccessful attempt to defuse tensions between the North and the South by reestablishing the Missouri Compromise line and extending it westward to the Pacific was involved in attempts by figures like James Buchanan who supported the administration's plan to extend the Missouri Compromise line (the 36°30' line) to the Pacific Ocean. However, the proposal to extend the line was repeatedly struck down by the House, highlighting the sectional divide and setting the stage for the Kansas-Nebraska Act. This Act, proposed by Stephen Douglas, aimed to organize the Nebraska territory, which resulted in the repeal of the Missouri Compromise line, further inflaming sectional tensions.