The simple answer is that the American public would not stand for such behaviour. The American President is not supposed to have that much power, and even the ones who have managed to acquire that level of power had to be circumspect in its use and either hide their abuses, or convince the public that the recipients were horrible people who definitely deserved it.
If a sitting president were to do something as blatant as simply throwing the opposition in jail on obviously false charges he would likely be impeached and removed from office (assuming he did not resign to avoid it as Nixon did).
And if, somehow, he had acquired enough power and blackmail material to avoid impeachment he would then likely have to deal with an armed and angry mob showing up on the Whitehouse lawn intent on giving a demonstration of the quaint American tradition of "tarring and feathering".
When was the Second Amendment last used to fight against the US government to protect people's freedom? Has some examples of violent protest at the local level when the government pushed its boundaries too far. I would expect unresolved national-level abuses would likely provoke similar responses.