United States President Donald Trump has gone on a clemency blitz, commuting the 14-year prison sentence of former Illinois Governor Rod Blagojevich and pardoning former New York Police Department Commissioner Bernie Kerik, among a long list of others.

Trump also told reporters that he has pardoned financier Michael Milken, who pleaded guilty for violating US securities laws and served two years in prison in the early 1990s. Trump also pardoned Edward DeBartolo Jr, the former San Francisco 49ers owner who was convicted in a gambling fraud scandal and who built one of the most successful National Football League teams in the game's history.

Blagojevich, who appeared on Trump's reality TV show, Celebrity Apprentice, was convicted of political corruption, including seeking to sell an appointment to Barack Obama's old US Senate seat and trying to shake down a children's hospital.

But Trump said he had been subjected to a "ridiculous sentence" that didn't fit his crimes.

Kerik served just over three years for tax fraud and lying to the White House while being interviewed to be secretary of the Department of Homeland Security.

"We have Bernie Kerik, we have Mike Milken, who's gone around and done an incredible job," Trump said, adding that Milken had "paid a big price".

Earlier, the White House announced that Trump had pardoned DeBartolo Jr, who was involved in one of the biggest owners' scandals in US football's history. In 1998, he pleaded guilty to failing to report a felony when he paid $400,000 to former Louisiana Governor Edwin Edwards in exchange for a riverboat gambling licence.

He also pardoned Ariel Friedler, a technology entrepreneur, who pleaded guilty to accessing a computer without authorisation; Paul Pogue, a construction company owner who underpaid his taxes; David Safavian, who was convicted of obstructing an investigation into a trip he took while he was a senior government official; and Angela Stanton, an author who served a six-month home sentence for her role in a stolen vehicle ring.

Poll-tested clemency

Blagojevich, a Democrat who hails from a state with a long history of pay-to-play schemes, exhausted his last appellate option in 2018 and had seemed destined to remain behind bars until his projected 2024 release date. His wife, Patti, went on a media blitz in 2018 to encourage Trump to step in, praising the president and likening the investigation of her husband to special prosecutor Robert Mueller's investigation into Russian meddling in the 2016 election - a probe Trump long characterised as a "witch-hunt".

Blagojevich's conviction was notable, even in a state where four of the last 10 governors have gone to prison for corruption. Judge James Zagel - who in 2011 sentenced Blagojevich to the longest prison term yet for an Illinois politician - said when a governor "goes bad, the fabric of Illinois is torn and disfigured".

Blagojevich became the brunt of jokes for foul-mouthed rants on wiretaps released after his December 9, 2008 arrest while still governor. On the most notorious recording, he gushes about profiting by naming someone to the seat Obama vacated to become president: "I've got this thing and it's f****** golden. And I'm just not giving it up for f****** nothing."

When Trump publicly broached the idea in May 2018 of intervening to free Blagojevich, he downplayed the former governor's crimes. He said Blagojevich was convicted for "being stupid, saying things that every other politician, you know, that many other politicians say". He said Blagojevich's sentence was too harsh.

A court reporter looks at a quote taken from a recorded conversation by former Illinois Governor Rod Blagojevich during his impeachment trial [Frank Polich/AP Photo]

Prosecutors have balked at the notion long fostered by Blagojevich that he engaged in common political horse-trading and was a victim of an overzealous US attorney, Patrick Fitzgerald. Fitzgerald said after Blagojevich's arrest that the governor had gone on "a political corruption crime spree" that would make Abraham Lincoln turn over in his grave.

Mueller - a subject of Trump's derision - was FBI director during the investigation into Blagojevich. Fitzgerald is now a private attorney for another former FBI director, James Comey, whom Trump dismissed from the agency in May 2017.

Trump also expressed some sympathy for Blagojevich when he appeared on Celebrity Apprentice in 2010 before his first corruption trial started. As Trump "fired" Blagojevich as a contestant, he praised him for how he was fighting his criminal case, telling him: "You have a hell of a lot of guts."

He later poll-tested the matter, asking for a show of hands of those who supported clemency at an October, 2019 fundraiser at his Chicago hotel. Most of the 200 to 300 attendees raised their hands, The Wall Street Journal reported, citing several people at the event.

Blagojevich testified at his 2011 retrial, describing himself as a flawed dreamer grounded in his parents' working-class values. He sought to humanise himself to counteract the blunt, profane, seemingly greedy Blagojevich heard on wiretap recordings played in court by prosecutors over several weeks. He said the hours of FBI recordings were the ramblings of a politician who liked to think out loud.

But jurors accepted evidence that Blagojevich demanded a $50,000 donation from the head of a children's hospital in return for increased state support, and extorted $100,000 in donations from two horse-racing tracks and a racing executive in exchange for quick approval of legislation the tracks' owners wanted.

He was originally convicted on 18 counts, including lying to the FBI, wire fraud for trying to trade an appointment to the Obama seat for contributions, and for the attempted extortion of a children's hospital executive. The 7th US Circuit Court of Appeals in Chicago in 2015 tossed out five of 18 counts against Blagojevich, including ones in which he offered to appoint someone to a high-paying job in the US Senate.

The appeals court ordered the trial judge to resentence Blagojevich, but suggested it would be appropriate to hand him the same sentence, given the gravity of the crimes. Blagojevich appeared via live video from prison during the 2016 resentencing and asked for leniency. The judge gave him the same 14-year term, saying it was below federal guidelines when he imposed it the first time.

Blagojevich had once aspired to run for president himself but entered the Federal Correctional Institution Englewood in suburban Denver in 2012, disgraced and broke. Court documents filed by his lawyers in 2016 portrayed Blagojevich - known as brash in his days as governor - as humble and self-effacing, as well as an insightful life coach and lecturer on everything from the Civil War to Richard Nixon. Blagojevich, an Elvis Presley fan, also formed a prison band called The Jailhouse Rockers.