During the deliberations over whether to ratify the US constitution in 1788, George Mason, the Virginian politician and scholar, warned against one particular inclusion: the right of a president to pardon people.
“He may frequently pardon crimes which were advised by himself,” Mason said. “It may happen, at some future day, that he will establish a monarchy, and destroy the republic.”
Donald Trump this week once more tested the boundaries of what voters will tolerate when it comes to presidential clemency, issuing pardons to 15 people.
They included two Republican politicians, four former private security contractors convicted of being involved in the deaths of Iraqi civilians, and two people convicted of crimes connected with the investigation into the president’s ties to Russia.
Mr Trump is hardly the first president to use his pardoning authority controversially, but historians say the outgoing president has acted unusually in how often he has used the power to benefit his own allies.
“Presidents tend to use the power of pardon to help bring the country together, to correct a miscarriage of justice, or where there are extenuating circumstances such as subsequent good behaviour,” said Barbara Perry, a presidential historian at University of Virginia.
“The extenuating circumstance is not supposed to be for the president themselves, it is not just supposed to be that this person is on their side,” she added.
Among those whom Mr Trump pardoned on Tuesday night were George Papadopoulos, a former low-level campaign adviser, and Alex van der Zwaan, a former lawyer. Mr Papadopolous was convicted of making false statements to investigators, while Mr van der Zwaan lied to investigators about his contacts with Rick Gates, a former senior adviser to Mr Trump.
Last month, the president also issued a pardon to Michael Flynn — his former national security adviser who admitted to lying about contacts with a Russian diplomat — and commuted the sentence of his former aide Roger Stone.
Constitutional experts said on Wednesday the president was operating within his legal powers. The constitution gives the president the authority to grant reprieves for offences against the US, though not for state crimes or those for which the offender could be impeached by Congress.
Mr Trump has the sole power to issue a federal pardon, although guidelines say it is ordinarily granted if a person accepts responsibility for a crime and has shown good behaviour for a significant period after conviction.
Pardons have proved controversial throughout history. Gerald Ford granted an unconditional and pre-emptive pardon to his predecessor Richard Nixon in an attempt to heal a divided country, although the move imperilled his own political career. Andrew Johnson caused far deeper anger in much of America in 1865 by issuing a blanket pardon for former Confederates.
There is also historical precedent for presidents using such powers to help themselves or those close to them. Bill Clinton caused fury when he pardoned both Marc Rich, the fugitive financier, and his own brother on his final day as president. George HW Bush pardoned several officials prosecuted for their roles in the Iran-Contra scandal.
But historians say these remain anomalies, and that Mr Trump stands out as unusual for the frequency with which he has issued pardons that appear to be in his own interest.
“As with a lot of other controversies during Donald Trump’s time in office, what is unusual here is the sheer scale,” said Tim Naftali, an associate professor of history at New York University.
Many expect Mr Trump to go further in his final weeks in office. The outgoing president has reportedly discussed granting pre-emptive pardons to his three children, as well as Jared Kushner, his son-in-law, and Rudy Giuliani, his personal lawyer. He could also choose to pardon others caught up in the Russia probe such as Paul Manafort, his former campaign chairman.
Some believe he may eventually feel constrained from doing so because of the legal view that a pardon automatically confers guilt. According to a Supreme Court ruling from 1915, a pardon “carries an imputation of guilt,” and acceptance of a pardon is “a confession of it”. Ford was said to have carried round a piece of paper in his wallet with those words written on it.
More important for Mr Trump’s own future however, is that by accepting a pardon, his associates then remove their right not to testify at a future trial for fear of implicating themselves.
According to an 1895 ruling by the Supreme Court, “[I]f the witness has already received a pardon, he cannot longer set up his privilege, since he stands with respect to such offence as if it had never been committed”. This may make them more likely to appear as a witness in any trial against Mr Trump himself, legal experts say.
There is one final option should Mr Trump wish to make sure he is not prosecuted once he becomes a private citizen: he could try to issue a pardon to himself.
No president has attempted this before, and the constitution does not mention the possibility, except to say that no one can be a judge in their own trial. Nixon weighed the option of pardoning himself, but eventually decided not to following warnings from his own advisers.
If Mr Trump were to try this, there would almost certainly be a legal challenge, one likely to end up in the Supreme Court.
“Even Nixon never tried this, and that was Nixon. But Donald Trump is different,” said Thomas Balcerski, associate professor of history at Eastern Connecticut State University. “Anyone who cares about the constitution and the state of our democracy has cause to be concerned.”