African-Americans and Republicans were facing several different problems during the 1870s. One of the most significant problems was the development of the Ku Klux Klan (KKK). This organization was developed in order to use violence to intimidate African-American citizens from using their newly gained rights during the Reconstruction era. The KKK used several different tactics (like lynching, destroying property) to prevent African-Americans from voting and participating in politics. These tactics hurt the Republicans as well, as African-Americans were usually Republicans, as Lincoln (who was Republican) was credited with freeing slaves.
Another problem faced by African-Americans were laws that limited their ability to vote. Poll taxes ( a fee to vote) and literacy tests (near impossible reading tests) were given to African-Americans to prevent them from voting. This was considered a legal loophole, as the constitution said that nobody can be stopped from voting based on race. Instead, states used these other means to stop African-Americans from voting. Again, this hurt Republicans as thousands of their party members were unable to vote.
Lastly, African-Americans struggled to obtain jobs during the 1870s. Due to their lack of education (as slaves weren't able to attend school) most African-Americans were still stuck on plantations as farmers, working for white-landowners.