Octavia, from PBS article...
Republicans' answer to the problem of the black vote was to add a Constitutional amendment that guaranteed black suffrage in all states, and no matter which party controlled the government. Congress spent the days between Grant's election and his inauguration drafting this new amendment, which would be the 15th added to the Constitution.
The writers of the Fifteenth Amendment produced three different versions of the document. The first of these prohibited states from denying citizens the vote because of their race, color, or the previous experience of being a slave. The second version prevented states from denying the vote to anyone based on literacy, property, or the circumstances of their birth. The third version stated plainly and directly that all male citizens who were 21 or older had the right to vote.
Determined to pass the amendment, Congress ultimately accepted the first and most moderate of the versions as the one presented for a vote. This took some wrangling in the halls of Congress, however. Many Congressmen felt that the first version did not go far enough, and that it left too many loopholes.
Congress passed the Fifteenth Amendment on February 26, 1869. But some states resisted ratification. At one point, the ratification count stood at 17 Republican states approving the amendment and four Democratic states rejecting it. Congress still needed 11 more states to ratify the amendment before it could become law.
All eyes turned toward those Southern states which had yet to be readmitted to the Union. Acting quickly, Congress ruled that in order to be let into the Union, these states had to accept both the Fifteenth Amendment and the Fourteenth Amendment, which granted citizenship to all people born in the United States, including former slaves. Left with no choice, the states ratified the amendments and were restored to statehood.
Finally, on March 30, 1870, the Fifteenth Amendment became part of the Constitution. To many, it felt like the last step of reconstruction. But just as some had predicted, Southerners found ways to prevent blacks from voting. Southern politics would turn violent as Democrats and Republicans clashed over the right of former slaves to enter civic life. White supremacist vigilante groups like the Ku Klux Klan gained strength as many whites refused to accept blacks as their equals. America still faced years of struggle.
https://www.pbs.org/wgbh/americanexperience/features/grant-fifteenth/
