The extension of slavery into the western territories.
For a specific instance, see the landmark supreme court case Dred Scott v. Sandford.
It confirmed segregation hope it helps
The KKK reemerged again and grew to millions of members in the 20s for a variety fo reasons.
They first gained a foothold in the depression after World War I before the roaring 20s. By stoking the xenophobia and conservative moralism of white Protestants in cities around the country (not just the South), the KKK was able to recruit members who feared immigrants and a loss of their "way of life."