Final answer:
The Reconstruction Republicans had their power significantly diminished due to political corruption, economic issues, and Supreme Court decisions in the 1870s.
Step-by-step explanation:
The group that had its power significantly diminished due to its association with political corruption, severe economic issues, and Supreme Court decisions in the 1870s was The Reconstruction Republicans (B). After the Civil War, the Republican Party implemented a series of reforms known as Reconstruction to rebuild the Southern states. However, the Democrats in the South, aided by paramilitary groups like the Ku Klux Klan, launched campaigns of violence and voter suppression to reclaim political control. Combined with economic turmoil and Supreme Court decisions that weakened federal support for Reconstruction, the power of the Reconstruction Republicans was diminished.