Sanford Levinson, commenting on the Libby pardon, reminds us that the Anti-Federalists were suspicious of the power to pardon:
"George Mason, a distinguished Virginian who refused to sign the Constitution, noted that '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.' Luther Martin, another non-signatory, also objected to the potential 'attempt [of the President] to assume to himself powers not given by the constitution, and establish himself in regal authority; in which attempt a provision is made for him to secure from punishment the creatures of his ambition, the associates and abettors of his treasonable practices, by granting them pardons should they be defeated in their attempts to subvert the constitution.'"
It was another reason to oppose a system of government that created a singular executive.