Final answer:
Ulysses S. Grant fought and weakened the power of the Ku Klux Klan during his presidency.
Step-by-step explanation:
The president who fought and weakened the power of the Ku Klux Klan was Ulysses S. Grant. During his administration, Grant used the authority given to him by the Enforcement Acts to arrest, indict, and prosecute Klansmen. He suspended habeas corpus in some South Carolina counties and worked towards restoring law and order in the South. Grant's actions led to a temporary decline in Klan activities and paved the way for fairer elections.