What does George Mason argue might happen if the president has the power to pardon people?

The President of the United States has the unrestrained power of granting pardon for treason; which may be sometimes exercised to screen from punishment those whom he had secretly instigated to commit the crime, and thereby prevent a discovery of his own guilt.

The text above was written by George Mason in September of 1787. He was essentially doodling the reasons that he would not sign the United States Constitution. His objection about the presidential pardon is a very sound one on the surface. Mason believes that the president, could, in theory, commit crimes through secret associates with the understanding that he would simply pardon those associates for the crime. This would allow the executive branch to usurp power from the other branches or even the citizens. Imagine if the president wanted to steal from the banks or the treasury to make himself wealthy. He could later pardon others for his crimes.

While the argument sounds valid at first, the fallacy of it is exposed when you consider Richard Nixon's Watergate fiasco. While he had others do his dirty work for him, he was actually going to be removed from office before resigning after his intrigue was discovered. Having said that, he would not have been able to pardon the perpetrators as he was going to be removed from office had he not resigned. In this way, the presidential pardon is checked by the ability of Congress to impeach the president.