During Reconstruction, Congress enacted the Civil Rights Act of 1870, also known as the Enforcement Act or the First Ku Klux Klan Act, in order to enforce the terms of the Fifteenth Amendment, which prohibited the states from denying anyone the right to vote based on race. The act provided criminal penalties for those attempting to prevent African Americans from voting by using or threatening to use violence or engaging in other tactics, such as making threats to terminate a person’s employment or evict them from their home.  As a result of the act, the criminal jurisdiction of the federal courts was expanded.

See also:

Spotlight on Judicial History: The South Carolina Ku Klux Klan Trials of 1871-1872