President Donald Trump on Thursday denounced his predecessor’s decision to shorten the prison sentence of WikiLeaks source Chelsea Manning. But he’s almost certainly unable to rescind the act of clemency that will see Manning released in May.

Pardons have been rescinded before and commutations have been modified, but experts appear to lean unanimously in Manning's favor, even though they are divided on the finer points of clemency-revocation powers.

"I can think of examples of where conditions were changed, but they're always toward leniency," professor and clemency expert P.S. Ruckman says.

“If the commutation was delivered and somehow there was an attempt to extend the punishment after the [date it specifies], I can think of no example of that happening – that would be bizarro-land,” Ruckman says.

One famous clemency modification cut a condition from Dwight Eisenhower's 1960 commutation of Maurice Schick's death sentence. Eisenhower said Schick, who raped and killed an 8-year-old girl, could never be released. President Gerald Ford in 1977 allowed for parole, which was quickly given.

Delivery has been a central issue in past pardon revocations, with delivery seen as making the clemency effective. A federal court found in 1869 that President Ulysses S. Grant could nix two pardons as they awaited delivery. But some experts dismiss the continued relevance of physical delivery in the internet age.

Fortunately for Manning – who Trump called an "ungrateful traitor" who "should never have been released" – her attorneys say clemency paperwork cutting short a 35-year sentence indisputably was delivered.

“Chelsea has the commutation order in hand, as do her attorneys and the prison,” says Chase Strangio, an American Civil Liberties Union attorney who represents Manning. “The commutation cannot be undone within the bounds of the law.”

Nancy Hollander, another attorney for Manning, says the signed order reached Manning in prison on Jan. 18, two days before Trump took office. “I made sure she had it in hand!” Hollander says.

Attorneys involved with past clemency actions agree Manning appears in the clear.

“It would be truly unprecedented in the modern era," attorney Henry Mazurek says about a hypothetical attempted commutation revocation.

"Except on the issue of delivery I don’t think there is any other legal precedent that would support that decision," he says.

Mazurek's client Isaac Toussie was at the center of a debate over the last pardon revocation, made in 2008 by President George W. Bush amid controversy one day after being announced.

Many experts believe Toussie's pardon could not be revoked, even if it was not physically delivered before Bush changed his mind.

“Once a grant of clemency has been signed by the president or otherwise made official, it cannot be rescinded,” says former U.S. Pardon Attorney Margaret Love, a position she believes applies to both the Toussie and Manning cases.

“I don't know of any authority who still subscribes to the notion that a pardon must be delivered in order to be effective – whatever 'delivered' means in an era of tweets," she adds.

Ruckman, however, continues to be believe the delivery window allows for withdrawing clemency, and that Toussie's pardon is not valid – something that never has been decided by a court.

Manning, formerly known by the first name Bradley, was arrested in 2010 after providing WikiLeaks a vast stash of diplomatic and military documents, badly embarrassing the U.S. government amid claims she put people’s safety at risk, though no deaths were attributed to the leak.

At trial, Manning was convicted in 2013 of most counts, including theft and espionage, but was acquitted of the most controversial charge, aiding the enemy, which was considered a major threat to reporting on classified documents. A military judge gave her a 35-year sentence.

Trump previously has insinuated Manning should be executed, and his most recent comments -- shared on Twitter -- appear to be a reaction to her latest column in The Guardian.

One scholar says Trump probably has few options to punish Manning other than finding another crime to charge her with, though doing so would be rife with questions of double jeopardy and statutes of limitation.