In the event a President is accused or found guilty of a serious crime while in office, can they pardon themselves for it?
It remains unclear if the President can pardon himself.
However, the Constitution gives broad leeway to who a President can pardon.
Article 2, Section 3, Clause 1 of the U.S. Constitution gives the President the right to pardon:
[The President] shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.
Consequences aside, the Constitution does not explicitly forbid a President to pardon himself, nor does it allow him to do so.
This has never happened before.
Its worth noting that former President Richard Nixon may have considered pardoning himself after the Watergate scandal. His Office of Legal Consul issued a ruling saying that he couldn't and in the end, he resigned and his successor, Gerald Ford pardoned him. Nixon wasn't indicted for the scandal. This ruling was never challenged in court, so any controversy would eventually end up in the Supreme Court.
More information in these articles:
If one accepts the premise of Antonin Scalia's judicial philosophy of "originalism," the Constitution is to be interpreted in the historical context of the intent of the Framers.
A power of self-pardon would give a President monarchial powers. While the Framers were undoubtedly familiar with the history of wholesale execution of Roman Senators by their Emperors (e.g., by Augustus), which is more likely: the Framers intended to give a President such power by omitting its prohibition in the text or that they couldn't imagine it?
The following scenario illustrates the self-evident absurdity of self-pardons: The President, anticipating Impeachment, calls a Joint Session of Congress. He strides in with his Constitutionally-protected AR15 and proceeds to mow down anyone who might vote to impeach him. Blowing the smoke off the barrel of his weapon, he declares "I pardon myself," the risk of the one exception explicitly stated in the text of The Constitution now eliminated.
There is no single answer
…as it depends on the nature of the crime, the Supreme Court's interpretation of the Constitution, on whether the president is "investigated", "accused", or "found guilty", and other factors.
This question recently (June 2018) became highly debatable after the president Trump's tweet claiming the right to pardon himself.
- As @Panda's answer says, the Constitution is vague about how to apply this right.
Law in the US […] is a game of precedent. When an element of law is vaguely written, as in the presidential pardon power, it remains largely undefined until it is "interpreted" (more accurately, assigned limits and meaning) by the courts. — source
- This answer has more insight, including links to articles written by prominent legal scholars;
- No US President has ever pardoned himself yet, so there is no exact precedent to refer to;
- We also know that the right to pardon is limited to Federal offenses, so the President has no right to pardon himself for breaking a State law;
- Also, it appears that the presidential pardon does not apply to crimes committed in another country;
One possible interpretation is that, as soon as the president pardons himself, the opponents would most certainly file the case to the Supreme Court.
The Court would review both the letter and the spirit of the Law. Obviously, if Trump declares in advance that he would pardon himself, this would render the entire prosecution a loss of time and taxpayers' money. And chances are, the court would rule that such "pardon in advance" goes against the spirit of the law and is illegal. Again, we don't know before it happens.
Another possible interpretation is based on the difference between pardoning someone who was found guilty and pardoning from investigation. The logic is fairly simple:
A president cannot be indicted while they remain in office. They must be impeached first. As soon as the President gets impeached, he loses the presidential authority to pardon anyone.
In other words:
A person in jail is no longer a President,
hence the President can not release himself from the jail,
hence he can not declare the intent to release himself,
hence there is no practical reason to give him right to void the prosecution.
Again, we don't know before it happens.