Roger Stone, whose jail term was commuted by Donald Trump, remains a "convicted felon and rightly so" according to former special counsel Robert Mueller.
Mr Mueller, whose investigation into alleged Russian interference in the 2016 presidential election triggered the attempted impeachment of the president, broke his nearly year-long silence to round on Stone, 67, one of Mr Trump's most controversial associates.
The Muller investigation led to 34 indictments with several key Trump allies serving jail time, including Paul Manafort the former chairman of his presidential campaign.
Mr Trump has repeatedly railed against the Mueller investigation and his decision to intervene on behalf of Stone, who was sentenced to 40 months in prison for lying to Congress and witness tampering, had been expected.
Writing on Twitter on Saturday, Mr Trump said that Stone was the victim of an "illegal Witch Hunt that never should have taken place."
Mr Mueller fought back in Washington Post op-ed in which he defended his investigation and Stone's conviction.
"Stone was prosecuted and convicted because he committed federal crimes. He remains a convicted felon, and rightly so," he wrote.
"The jury ultimately convicted Stone of obstruction of a congressional investigation, five counts of making false statements to Congress and tampering with a witness.
"Because his sentence has been commuted, he will not go to prison. But his conviction stands."
Stone was one of the key scalps claimed by the Mueller investigation into allegations of Kremlin involvement in the 2016 election.
Mr Trump and his allies have insisted that Democrat complaints of Russian interference were a hoax designed to undermine his presidency.
Two Republican senators have already spoken out against Mr Trump's intervention.
Utah senator, and former presidential candidate, Mitt Romney attacked the move on Twitter.
"Unprecedented, historic corruption: an American president commutes the sentence of a person convicted by a jury of lying to shield that very president," he wrote on Saturday morning.
Pennsylvania senator Pat Toomey argued the court's decision should have been respected.
Do RINO’S Pat Toomey & Mitt Romney have any problem with the fact that we caught Obama, Biden, & Company illegally spying on my campaign? Do they care if Comey, McCabe, Page & her lover, Peter S, the whole group, ran rampant, wild & unchecked - lying & leaking all the way? NO!
— Donald J. Trump (@realDonaldTrump) July 12, 2020
"The president clearly has the legal and constitutional authority to grant clemency for federal crimes.
"However, this authority should be used judiciously and very rarely by any president.
"While I understand the frustration with the badly flawed Russia-collusion investigation, in my view, commuting Roger Stone's sentence is a mistake."
Their criticism incensed Mr Trump who, on Twitter, accused the two men of being "RINOS," a pejorative meaning "Republican in name only."