Article II, Section 2, Clause 1:

The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any Subject relating to the Duties of their respective Offices, and he shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.

The Constitution establishes the President’s authority to grant clemency, encompassing not only pardons of individuals but several other forms of relief from criminal punishment as well.¹ The power, which has historical roots in early English law,² has been recognized by the Supreme Court as quite broad. In the 1886 case Ex parte Garland, the Court referred to the President’s authority to pardon as unlimited except in cases of impeachment, extending to every offence known to the law and able to be exercised either before legal proceedings are taken, or during their pendency, or after conviction and judgment.³ Much later, the Court wrote that the broad power conferred in the Constitution gives the President plenary authority to ’forgive’ [a] convicted person in part or entirely, to reduce a penalty in terms of a specified number of years, or to alter it with certain conditions.⁴

Despite the breadth of the President’s authority under the Pardon Clause, the Constitution’s text provides for at least two limits on the power: first, clemency may only be granted for Offenses against the United States,⁵ meaning that state criminal offenses and federal or state civil claims are not covered.⁶ Second, the President’s clemency authority cannot be used in Cases of impeachment.⁷

Beyond textual limits, certain external constitutional and legal considerations may act as constraints on the power. For instance, the Court has indicated that the power may be exercised at any time after [an offense’s] commission,⁸ reflecting that the President may not preemptively immunize future criminal conduct. In Schick v. Reed, the Court recognized that an exercise of clemency may include any condition which does not otherwise offend the Constitution,⁹ suggesting that the President may not make clemency subject to a condition that is prohibited by another constitutional provision.¹⁰ Other apparent limitations include not affecting vested rights of third parties, such as where forfeited property is sold,¹¹ or proceeds paid into the treasury, which can only be secured to the former owner . . . through an act of [C]ongress.¹² The Court in The Laura also alluded to an exception for fines . . . imposed by a co-ordinate department of the government for contempt of its authority,¹³ though a later case recognized that the President may pardon one who is subject to criminal punishment for contempt of court.¹⁴

Assuming the recognized limitations are not transgressed, a full pardon granted by the President and accepted by its subject¹⁵ prevents or removes any of the penalties and disabilities consequent upon conviction . . . .¹⁶ In several nineteenth-century cases, the Supreme Court suggested that a pardon broadly obviates all legal guilt of the offender, effectively erasing the crime from existence.¹⁷ Subsequent cases appear to have backed away from this understanding,¹⁸ suggesting instead that, although a full pardon precludes punishment for the offense in question, a prior and pardoned offense may still be considered in subsequent proceedings.¹⁹

Congress generally cannot substantively constrain the President’s pardon authority through legislation, as the Court has held that the power of the President is not subject to legislative control. Congress can neither limit the effect of his pardon, nor exclude from its exercise any class of offenders. The benign prerogative of mercy reposed in him cannot be fettered by any legislative restrictions.²⁰ Nevertheless, there is historical precedent for legislation facilitating the exercise of the pardon power through funding of Executive Branch positions to review clemency petitions.²¹ Congress also has other constitutional tools that it may use in relation to the President’s pardon authority, provided the legal conditions associated with those tools are met, such as oversight,²² impeachment,²³ and constitutional amendment.²⁴

Source:

https://constitution.congress.gov/browse/essay/artII-S2-C1-3-1/ALDE_00013316/