So a big issue with reconstruction, separate from a waning appetite for it in the north amid southern resistance and economic stress, was the supreme court of thelate 1860s to the1890s effectively rolling back reconstruction's gains. The 14th amendment was effectively neutered, the civil rights act of 1875 was declared unconstitutional, and famously codified separate but equal, among other rulings allowing Jim Crow to come into being.
Now many of these justices on the court who would make these rulings were Republican appointees. Samuel Freeman Miller and Stephen Johnson Field were appointed by Lincoln. Morrison Waite and Joseph P. Bradley were appointed by Grant. William Burnham Woods was nominated by Hayes.
All of these were justices who pariticipated in the rollback of reconstruction and providing the legal veneer for Jim Crow to be established. However, what if Lincoln, Grant, and Hayes had better SCOTUS appointments? They were certainly capable of appointing justices who would uphold reconstruction's gains-Hayes appointee John Marshall Harlan famously was the lone dissenter in Plessy V. Ferguson and Civil Rights Cases that struck down the 1875 Civil Rights Act.