The correct answer to this open question is the following.
The group that worked to reduce the power of political machines during the late 19th and early 20th century were progressive reformers.
Progressive reformers were the ones that confronted political machines in an intent to stop their corruptive practices in the United States.
Political machines were powerful organizations in the political parties that were headed by wealthy people that tried to control the political agenda and influence officials in the government. They wanted to control towns, counties, cities, and states. On e of the most corrupt political machines in the mid-1800s was Tammany Hall, led by William Tweed.
