Let’s review what has been built over the last years and decades.
There is the dragnet surveillance program at the NSA, built by Bush and expanded by Obama, which might be used to monitor political dissidents and blackmail them with personal indiscretions. (It has been restrained, but not by much.)
There is Obama’s drone assassination program, in which the president personally condemns people, including a U.S. citizen, to death without trial. It could be used domestically to pick off those who won’t submit to other pressure.
There is the gigantic U.S. prison system. As of 2014, some 6,851,000 people were under criminal supervision (either in prison, jail, or on probation) in the United States. That’s over 2 percent of the entire population. In many places, the criminal justice system is used as an all-purpose warehouse for any sort of social dysfunction, or simply as a way to fund the local government. Some rural locations have a supervision rate of about 10 percent. Even 2 percent is likely more than enough capacity than would be necessary to utterly crush any organized political movement — the Soviet gulags took up roughly 1-2 percent of the Soviet population at their height.