clause that exempted citizens from other voting requirements if their grandfathers had been registered voters. The Court also stated that the amendment does not confer the right of suffrage, but it invests citizens of the United States with the right of exemption from discrimination in the exercise of the elective franchise on account of their race, color, or previous condition of servitude, and empowers Congress to enforce that right by "appropriate legislation". The Court wrote: White supremacists, such as the Ku Klux Klan (KKK), used paramilitary violence to prevent blacks from voting. A number of blacks were killed at the Colfax massacre of 1873 while attempting to defend their right to vote. The Enforcement Acts were passed by Congress in 1870–1871 to authorize federal prosecution of the KKK and others who violated the amendment. However, as Reconstruction neared its end and federal troops withdrew, prosecutions under the Enforcement Acts dropped significantly. In United States v. Cruikshank (1876), the Supreme Court ruled that the federal government did not have the authority to prosecute the perpetrators of the Colfax massacre because they were not state actors. Congress further weakened the acts in 1894 by removing a provision against conspiracy. In 1877, Republican Rutherford B. Hayes was elected president after a highly contested election, receiving support from three Southern states in exchange for a pledge to allow white Democratic governments to rule without federal interference. As president, he refused to enforce federal civil rights protections, allowing states to begin to implement racially discriminatory Jim Crow laws. A Federal Elections Bill (the Lodge Bill of 1890) was successfully filibustered in the Senate. Post-Reconstruction From 1890 to 1910, poll taxes and literacy tests were instituted across the South, effectively disenfranchising the great majority of black men. White male-only primary elections also served to reduce the influence of black men in the political system. Along with increasing legal obstacles, blacks were excluded from the political system by threats of violent reprisals by whites in the form of lynch mobs and terrorist attacks by the Ku Klux Klan. Some Democrats even advocated a repeal of the amendment, such as William Bourke Cockran of New York. In the 20th century, the Court began to read the Fifteenth Amendment more broadly. In Guinn v. United States (1915), a unanimous Court struck down an Oklahoma grandfather clause that effectively exempted white voters from a literacy test, finding it to be