To Jeffrey Toobin, author of The Pardon: The Politics of Presidential Mercy, pardons are “X-rays into the soul” of the American president who gives them, revealing true character. Pardons can show compassion and mercy in the occupant of the Oval Office. More often, they expose venality and self-preservation.

Toobin said: “One thing you can say about Donald Trump is that his moral compass always points in the same direction, and his motives are always the same, which are transactional and narcissistic. This is a good example, I think, of my thesis that pardons are X-rays into the president’s soul.”

In his first term, Trump “wanted to settle a score with Robert Mueller, so he pardoned everyone Mueller prosecuted” in the special counsel’s investigation of Russian election interference in 2016 and links between Trump and Moscow, Toobin said.

“Trump wanted to take care of his family, so he pardoned his daughter’s father-in-law, Charles Kushner,” who is now nominated as US ambassador to France, the author added. “He wanted to reward his House Republican allies, so he pardoned several who were engaged in egregious corruption, and he pardoned people who were [his son-in-law and adviser] Jared Kushner’s friends.”

Asked why he wrote his 10th book to come out now, so soon after such a momentous election, Toobin, a former CNN legal analyst and New Yorker writer, said: “I saw that from a very early stage in the campaign Trump was talking about January 6 pardons. But I also recognized that if Kamala Harris won, there would be pressure on her to pardon Trump” on 44 federal criminal charges now dismissed.

“I think the proper way to understand the January 6 pardons [issued on day one of Trump’s second term] is to remember that Trump himself was a January 6 defendant, Toobin said. “He wasn’t charged with the riot the way the others were, but he was charged with trying to overthrow the election with the fake electors scheme. And if you look at the way in the beginning part of his second term he is settling scores and rewarding his friends, the January 6 pardons told you exactly how he was going to go about conducting his administration.”

Reportedly saying: “Fuck it, release ’em all”, Trump gave pardons, commutations or other acts of clemency to the absurd, such as the J6 Praying Grandma and the QAnon Shaman, and to the outright sinister: hundreds who attacked police, militia leaders convicted of seditious conspiracy, Toobin wrote.

He said: “If Trump had tried to carve out the non-violent January 6 rioters [for clemency], that that would have been somewhat more defensible than what he wound up doing, which was, in my view, completely indefensible.”

His point about pardons being an X-ray for the soul applies to Joe Biden too.

On the page, Toobin decries the 46th president’s decision to pardon his son, Hunter Biden, on gun and tax charges and any other grounds, having said he would not do so.

Toobin said: “When you think about Hunter, this is a guy who was convicted of a crime, who pleaded guilty to other crimes. So it’s not like these were made-up accusations against him. Yes, the criminal justice system came down hard on him, but the criminal justice system comes down hard on a lot of people, and their father wasn’t president of the United States, so they don’t get this kind of break. And I just think that’s not how the system is supposed to work.”

Publishing schedules being what they are, The Pardon does not cover the last-minute pre-emptive pardons Biden gave his brothers, his sister and their spouses, as well as public figures held to be in danger of persecution by Trump, Liz Cheney and Gen Mark Milley among them.

But Toobin told the Guardian: “The family pardons were just bizarre, because these people, as far as I’m aware, are not even under investigation. But [Biden] was so worried and fixated on his family that he took this extraordinary step, which is just egregious to me.”

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The pardon is older than America. British kings could pardon people. When the states broke away, they kept the pardon for presidents. George Washington used it after the Whiskey Rebellion of 1794, for men convicted of treason. Abraham Lincoln used it during the civil war to reprieve Union soldiers sentenced to die and to forgive Confederates in the name of peace.

Such acts of mercy continue, memorably including Jimmy Carter’s clemency for those who dodged the draft for Vietnam and Barack Obama’s record-setting issue of commutations for people mostly jailed for minor crimes. Even Trump handed down mercy in his first term, amid the push which produced the First Step Act, criminal justice reform he swiftly seemed to forget.

Asked which modern president has best used the pardon power for the public good, Toobin picks Obama. Inevitably, though, most public attention falls on use of the power for controversial ends, including George HW Bush’s mop-up of the Iran-Contra scandal and Bill Clinton’s last-minute pardon for Marc Rich, a financier turned fugitive.

The most famous pardon of all, the one Gerald Ford gave Richard Nixon after the Watergate scandal, hangs over every president. As Toobin sees it, had Harris taken office in January, pressure to pardon Trump of his alleged federal crimes would have been great, and it would have sprung from “an interesting shift in the conventional wisdom” about Ford and Nixon.

“It was widely considered a disaster in 1974” – Carl Bernstein told Bob Woodward, his Washington Post partner in reporting Watergate, “The son of a bitch pardoned the son of a bitch” – “but now you’ve had Ted Kennedy giving Gerald Ford an award, saying he was right about the pardon. You have Bob Woodward changing his mind [to say the pardon was ‘an act of courage’]', and at the oral argument of the Trump v United States supreme court case [about presidential immunity, last April], Justice Brett Kavanaugh said, ‘Well, everyone now agrees Ford did the right thing.’”

Toobin thinks Ford did the wrong thing, given Nixon’s clearly criminal behavior. He was also “struck by the absence of a book heavily focused on that issue of the Ford pardon. So all those combinations led me to try to not only write a book, but have it come out in early 2025.”

He duly devotes most of that book to the Nixon pardon: how Ford agonized about it, decided to do it, then employed an obscure young lawyer to make sure Nixon took it.

“I had certainly never heard of Benton Becker when I went into this,” Toobin said. “And I think his central role illustrates how ill-prepared Ford was for the whole issue of dealing with Nixon, because if you want to address an issue that will be the central event of your presidency, maybe you want to entrust it to someone who is not a young volunteer lawyer, who is himself under criminal investigation.

“Now, if you say that, you should say that Becker [who died in 2015] was completely cleared. But it struck me as ludicrous that a president with the entire resources of the White House counsel’s office, the justice department and the entire American government, chose to invest so much authority in this young man. I think that just illustrates how Ford’s anxiousness to get the whole Nixon subject behind him led him to fail to consider the consequences of what he was doing.”

The rights and wrongs of the Nixon pardon echo to this day. Looking again to last year’s supreme court arguments over presidential immunity, which the justices decided did apply in relation to official acts, Toobin said: “I thought the best question at that oral argument was Justice [Ketanji Brown] Jackson saying, ‘If presidents are immune, why did Ford need to pardon Nixon?’ Which is a great question, and doesn’t really have an answer. The only real answer is that [Chief Justice] John Roberts just completely changed the rules” in Trump’s favor.

The Pardon is Toobin’s guide to how presidential pardons work, for good or often ill. He is not optimistic that the power can be reined in or usefully reformed:

“The both good and bad news is that our constitution is almost impossible to amend, and no one cares enough about pardons one way or the other to undertake the massive task of of trying to amend the constitution. It’s not even clear how you would amend it. My solution to pardon problems is not changing the constitution, it’s getting better presidents.”

That will have to wait – at least for four more years.

• The Pardon is out now