Answer:
Popular Sovereignty
Explanation:
The Progressive Era was a period of social activism and political reform in which many civilians participated (direct democracy) in order to reform the vertical (hierarchy) political system. Among the targets were the political machines with their corrupt and malfunctioning representatives.
Popular Sovereignty, that dates back to the Enlightenment, is the purest of (and most horizontal) democratic government, in the sense that the goverment´s purpose should be to protect (the civil rights) and serve the people, and not the other way around. And if they fail to do, or are corrupt or both, so they should be replaced.
It would be "Whether a new state or territory should enter the Union as Free or Slave" that had the most impact on the Civil War, since the North was strongly against the spread of slavery, while the South was in favor of it.
The Atlantic Slave Trade, hope this helps
