Donald Trump's commutation of the prison sentence of longtime ally Roger Stone was a case of "unprecedented, historic corruption," Senator Mitt Romney tweeted Sunday, making his a rare Republican voice raised in criticism of the president.

Stone had been set to go to jail on Tuesday after his conviction of seven felony crimes. They include tampering with a witness and obstructing the House investigation into whether the Trump campaign conspired with Russia to help him win the 2016 election.

Romney, who infuriated Trump when he became the only Republican to vote to convict the president in his impeachment trial, pulled no punches on Saturday. "Unprecedented, historic corruption: an American president commutes the sentence of a person convicted by a jury of lying to shield that very president," he tweeted.

His blunt criticism set him apart from most Republicans, who remained largely mute on the matter, but it aligned him with the unanimous condemnation coming from the president's Democratic critics, led by House Speaker Nancy Pelosi.

'Staggering corruption'

"President Trump's decision to commute the sentence of top campaign advisor Roger Stone, who could directly implicate him in criminal misconduct, is an act of staggering corruption," she tweeted early Saturday.

Pelosi called for legislation "to ensure that no president can pardon or commute the sentence of an individual who is engaged in a cover-up campaign to shield that president from criminal prosecution." Romney, who was the Republican nominee for president in 2012, has been shunned by some in the party since his vote on impeachment. The president's oldest son, Donald Trump Jr., called for him to be "expelled" from the party.

Trump's action late Friday immediately brought new charges that the president has intervened freely in the US justice system to help friends and allies and punish critics and perceived enemies. In May, the US Justice Department moved to dismiss its own case against Michael Flynn, a former national security advisor to Trump, though he had pleaded guilty to lying to the FBI. A federal judge has demanded a further review of that highly unusual administration move.

But Stone was the first person directly involved in Trump's campaign to receive clemency. On Friday, the White House defended the commutation order benefiting Stone in language reminiscent of Trump's frequent tweets, saying "overzealous prosecutors" had pursued Stone based on charges stemming from the "Russia hoax" and political "witch hunts."

The statement did not claim that Stone was innocent of the charges facing him but said he should have a proper chance to clear his name. But leading Democrats said the commutation was a perversion of the American legal system. Representative Adam Schiff, the Democrat who led the impeachment drive against Trump, put it bluntly: "With Trump there are now two systems of justice in America: One for Trump's criminal friends and one for everyone else."