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.

to act quickly in times of danger to the united states

because you have to act quickly if you want to live/survive.

it increase by 12% a year from 1812-1820