This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
The Constitution provides that the President " shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment."
31 92 U. S. 105; 23 L. ed. 605.
32 For a further discussion of this subject see speech of Senator A. O. Bacon, Jan. 13, 1909, Cong. Rec. vol. 43, p. 1011ff. See also debate in the Senate, March 3, 1909, Cong. Rec. vol. 43, p. 3813ff.
33 Chapter XXXIX (The Process Of Legislation As Constitutionally Determined. 248. Constitutional Provisions).
This pardoning power, like the veto power, has given rise to very few constitutional questions. It will be seen that the power is limited to offences against the United States. Cases of impeachment are expressly excepted from its reach, and we shall later consider whether it may extend to the remission of penalties imposed for civil contempts of court.
The effect of a pardon is to obliterate the offense, but it does not operate to impair the rights of others, as for example, to restore the offender's property which has been forfeited ;34 nor does it restore one ipso facto to a forfeited office.35 Also, though the pardon takes away the guilt, it does not affect the fact of conviction of the crime, which fact may be later shown as bearing upon the offender's character.
The power to pardon includes the right to remit part of the penalty as well as the whole, and in either case may be made conditional. The power may be exercised at any time after the offense has been committed, that is, either before, during, or after legal proceedings for punishment.36 General pardons, granting amnesty to classes of offenders, without naming them individually, may be granted.37
 
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