So is that the end of the matter? No. The Constitution gives no government official uncheckable power. If the president abuses his law-enforcement and clemency powers, if he wields them corruptly, Congress is authorized to impeach him. Like the courts, Congress has power neither to rescind a pardon once granted, nor to direct the executive branch to pursue a case the president wants terminated. But unlike the courts, Congress has the power to impeach and remove the president — thereby divesting him of the capacity to abuse the awesome powers of the executive.
Impeachment is the only context in which it is sensible to discuss President Trump’s warping of law-enforcement processes on behalf of political allies such as Arpaio and Flynn. A president may not be indicted for exercising authority he unquestionably has, no matter how disagreeable or even repugnant we may find the exercise. But if it is a corrupt exercise, he may be impeached over it.
Still, to repeat a point made many times in these pages, and in Faithless Execution, my book on the subject (clearly more timely now than when it was published!), impeachment is a political remedy, not a legal one.