There were a very large number of events that shaped the political debate over slavery. All of these events helped to push the North and the South further apart on the issue.
Many of the events were not about slavery per se but about the extent to which slavery would be able to expand out beyond where it currently existed. This was the issue that was settled in the Missouri Compromise after the Louisiana Purchase. It is also the issue that was reopened by the Mexican-American War. It was then settled by the Compromise of 1850, only to be reopened by the Kansas-Nebraska Act.
However, some of the events that happened were really more closely tied to slavery and the issue of rights for African Americans. Perhaps the most important example of such an event is the passage of the Fugitive Slave Act as part of the Compromise of 1850. That law put slavery, and the issue of what sort of treatment of escaped slaves was just and humane, in the national spotlight. Another such incident was the Dred Scott case. In this case, the Supreme Court ruled, among other things, that African Americans had no rights and could not be citizens. This, too, put the issue of slavery and how blacks should be treated in the spotlight.
These issues, among others, served to shape the political debate over slavery.