Final answer:
Northern support for Reconstruction declined during the 1870s due to ongoing problems in the South, the shift in political control to the Democratic Party, and the economic depression following the Panic of 1873.
Step-by-step explanation:
The decline in Northern support for Reconstruction during the 1870s can be attributed to several factors. Firstly, the ongoing problems in the South, such as violence and corruption, caused voters to turn against the Republican Party in the midterm elections of 1874. The Democratic Party gained majority control in Congress, signaling a shift in priorities. Additionally, the Panic of 1873 resulted in an economic depression that diverted attention away from Reconstruction.