Final answer:
The Republican Party's post-Civil War base included new Black male voters and white Northerners, strengthening its presence in the South through policies favorable to African Americans and the advocacy of Radical Republicans for greater equality.
Step-by-step explanation:
After the Civil War, the voting base of the Republican Party comprised mainly of new Black male voters and white Northerners who supported Reconstruction. The Republican Party, known as the party of Lincoln, had earned the support of African Americans in the South, where they were newly enfranchised, as well as in the Northeast due to their efforts to provide Blacks with greater legal rights. During Reconstruction, Radical Republicans who advocated for a more equal society pushed for measures such as the Fifteenth Amendment, which granted Black men the right to vote, to build a solid Republican base in the South.
Despite challenges from the local Democrat Party and the emergence of paramilitary groups seeking to prevent the enfranchisement and political participation of former slaves, the Republican Party managed to maintain a significant following. However, by the end of 1870, Reconstruction efforts were facing severe backlash, and the subsequent contested presidential election and the withdrawal of federal troops signaled the failure to establish an interracial democracy with equal rights for all citizens. Democrats, largely the party that had supported slavery and opposed African American rights, regained control in the South after 1877, as Reconstruction policies were dismantled.
Although the black vote was critical in local Northern elections, nationally, both parties exhibited indifference to the needs of African Americans, often subordinating these concerns to gain the support of Southern white voters. Nevertheless, during Reconstruction, it was the new Black male voters and white Northerners who formed the core support for the Republican Party.