Final answer:
Stephen A. Douglas introduced the bill that organized the Kansas and Nebraska territories in 1854, known as the Kansas-Nebraska Act, which contributed to the collapse of the Second Party System and heightened tensions over slavery.
Step-by-step explanation:
In 1854, Stephen A. Douglas introduced a bill into Congress to organize the Kansas and Nebraska territories. This piece of legislation became known as the Kansas-Nebraska Act. Douglas's bill effectively repealed the Missouri Compromise by allowing these territories to decide on the issue of slavery through popular sovereignty. This approach led to increased sectional tension and was a crucial factor in the collapse of the Second Party System. The political landscape shifted dramatically, giving rise to the Republican Party and intensifying conflict over slavery that would eventually lead to the Civil War.