What factors contribute to the increased rigidity of racial segregation and renewed white efforts to exclude blacks from political life in the late 1800s?

One of the main factors is the return to power of white Democratic Party politicians in state legislatures soon after the Reconstruction. At that time, the Democrats were pro-slavery and pro-segregation (President Lincoln is a Republican). Those white pro-slavery democrats want to restrict the rights of now free blacks and so worked hard to pass the so-called Jim Crow laws in state and local governments, and they formed militias to intimidate black voters and political opponents.

In 1857 the courts ruled that African Americans were not even considered citizens of the United States. Whites in the northern states had excluded black people from sitting on public transportation and didn't allow them in hotels and restaurants unless they were servants. The few times they were allowed into theaters their seats were separate from whites. Their schools and churches were also segregated from white buildings. Jim Crow laws took effect for most of the latter half of the 1800s and finally became overturned by the courts in 1883.