What was the main issue that politicians were fighting over during the time period leading up to the Civil War?

Whether slavery could exist in newly acquired territories.

What was the Kansas-Nebraska Act of 1854, and what principle did it employ to decide the slavery question in those territories?

The Kansas-Nebraska Act divided the Louisiana Purchase into two territories and used popular sovereignty, allowing residents to decide on slavery.

Bleeding Kansas was a period of violence between pro-slavery and anti-slavery factions in Kansas, caused by the popular sovereignty decision.

Thousands of pro-slavery Missourians illegally voted in Kansas, leading to a fraudulent election.

Two rival state legislatures were established, one pro-slavery in Lecompton and one anti-slavery in Topeka.

I can identify the causes and effects of the Civil War.

1
According to the Kansas-Nebraska Act of 1854, the territories of Kansas and Nebraska would decide whether to allow slavery based on popular sovereignty, which meant the people living in those territories could decide for themselves.
2
The Dred Scott decision stated that enslaved people were not citizens and therefore had no right to sue in federal court, and also that slave owners could take their "property" anywhere they wanted without fear of being deprived of it, opening any territory or state in the Union to slavery.
3
The Republican Party was formed in 1854 from a diverse group including former members of the Know Nothing Party, abolitionists, free soilers, and Conscience Whigs, who argued that slavery should not be able to spread into new territories.