WASHINGTON - United States President Donald Trump's claim over the weekend that he has the "complete power to pardon", in response to the investigation of possible ties between his 2016 election campaign and Russia, has sparked a debate on whether he could ultimately use the power to pardon himself.

Trump's assertion was made in a series of early morning messages on Twitter on Saturday (July 22). "While all agree the US President has the complete power to pardon, why think of that when only crime so far is LEAKS against us," he wrote on Twitter.

His statement came after The Washington Post reported on Friday that Trump and his legal team had discussed the possible use of presidential powers to pardon aides, family members and possibly even himself as a way to limit or undercut the investigation overseen by special counsel Robert Mueller.

The questions have heightened interest in several key legal issues about the scope and limits of Trump's powers. Here is what you need to know.

1. What is the pardon power?

The US Constitution gives Presidents clemency powers "to grant reprieves and pardons for offenses against the United States, except in cases of impeachment". By bestowing a pardon on someone who committed a federal crime, the president nullifies the legal consequences of that crime.

According to the BBC, in practice, this means the President can offer clemency, a reduced sentence or a full pardon (which essentially officially forgives a person for a crime).

A person does not have to be charged or convicted to receive a pardon and individuals cannot be pardoned for state-level crimes.

2. How would pardons affect the Trump-Russia investigations?

What this means in the context of the Russian investigation is that Trump could grant immunity to his aides, family members or anybody else believed to be the targets of Mueller's investigation.

Anyone pardoned by Trump would receive immunity from being charged with a federal crime over his or her past conduct covered by the pardon. That could contract the scope of Mueller's investigation.

3. Can Trump pardon himself?

This is not clear. The only limitation explicitly stated in the Constitution is a ban on using a

pardon to stop an impeachment proceeding in Congress, and the only obvious implicit limitation is that he cannot pardon offences under state law. But some legal scholars think a President cannot pardon himself either, because it would be a conflict of interest.

In August 1974, four days before the resignation of President Richard Nixon, the Justice Department's Office of Legal Counsel (OLC) issued a terse legal opinion stating that "it would seem" that Nixon could not pardon himself "under the fundamental rule that no one may be a judge in his own case".

The OLC's guidance, however, does not explain that principle has been transformed into an unwritten legal limit on the power the Constitution bestows on Presidents.

Legal experts are also divided on the issue.

In a commentary in the Washington Post on Friday, two former White House chief ethics lawyers under former President Barack Obama and President George W. Bush, along with a Harvard Law professor, cited the OLC's legal guidance to make the argument that Trump could not do so.

"The Constitution specifically bars the President from using the pardon power to prevent his own impeachment and removal," they wrote.

"It adds that any official removed through impeachment remains fully subject to criminal prosecution. That provision would make no sense if the President could pardon himself."

Other legal specialists have come out the other way. In a 1998 House Judiciary Committee hearing about the then-proposed impeachment of President Bill Clinton, for example, Republican Representative Bob Goodlatte, who is now chairman of that panel, stated: "The prevailing opinion is that the President can pardon himself."

There is no definitive answer because no President has ever tried to pardon himself and then been prosecuted, which would give courts a chance to weigh in. If Trump did purport to pardon himself, and was later indicted anyway, it could create an opportunity for the Supreme Court to resolve the question.

4. Could pardons increase Trump's legal risk?

Some legal specialists think so. While pardons are widely understood to be irrevocable even if obtained through questionable means, some experts think that a President who abuses his pardon power might be subject to prosecution.

In 2001, then Senator Jeff Sessions, who is now Trump's attorney-general, voiced support for the idea of a bribery investigation into President Clinton for his pardon of Marc Rich, a fugitive financier whose ex-wife had donated to the Democratic Party and the Clinton library foundation.

In an op-ed published in The New York Times on Friday, two University of Chicago law professors argued that if Trump pardoned his relatives and aides to cover up possible crimes and impede Mueller's investigation, rather than for reasons of mercy or public welfare, it could increase the risk that Trump is later charged with obstruction of justice.

Trump's previous actions, including purportedly pressuring James Comey, then the director of the FBI, to back off the investigation into Flynn, have already raised that spectre.

SOURCES: BBC, NYTimes, Washington Post