Sen. John McCain, powerful chairman of the Armed Services Committee and former Republican presidential nominee, is riding to the rescue of retired Gen. David Petraeus, who is facing a retroactive demotion that McCain says could be illegal.

In a letter to Secretary of Defense Ash Carter, McCain cited “troubling media reports” that Carter might strip Petraeus of his fourth star, a rare form of discipline that could violate administrative law and, McCain argued, would be “manifestly unreasonable and unfair.”

The Daily Beast first reported last week that Carter was considering the action, in light of Petraeus admitting to sharing classified information with his mistress and biographer, Paula Broadwell. Carter is said to view a demotion as sending a message throughout the military that even the most revered officers aren’t immune from punishment for their actions.

A senior defense official told The Daily Beast that McCain has also posed questions to the secretary’s office about how Petraeus’s case was handled and why the military is considering punishing him now—a year after he pleaded guilty to mishandling classified information and nearly four-and-a-half years after the offense itself.

A spokesperson for McCain’s office declined to discuss the matter beyond what was in McCain and Sen. Jack Reed’s public letter, sent last week. “I’m not going to comment on staff correspondence,” the spokesperson said.

In their letter, McCain and Reed, a Democrat from Rhode Island, questioned not only whether stripping Petraeus of his star was fair but if it was legal.

“It is rare that a retirement grade determination is conducted for an officer previously retired from the U.S. armed forces,” McCain and Reed wrote, referring to the process by which the military can reduce an officer’s rank even after it has been awarded.

Petraeus retired from the U.S. Army in August 2011 to become the director of the Central Intelligence Agency. Three days before he switched careers, Petraeus gave Broadwell eight notebooks he’d compiled that contained highly classified information and other secrets, including the identities of covert officers, intelligence capabilities, quotes from high-level meetings of the National Security Council, and notes about Petraeus’s discussions with President Obama.

The senators said Carter’s decision was “discretionary” and argued that “it is well established in law” that rank determinations can only be reopened if the military discovered fraud on the part of the officer, some error in the decision to award the rank in the first place, or if there was “newly discovered evidence” of wrongdoing “presented within a reasonable time….”

Carter has publicly sought to distance himself from the controversy, saying during a trip to Europe last week that he hadn’t yet reached a conclusion.

Defense officials said that the decision to demote Petraeus rests solely with the secretary.

“My guess is the lawyers are carefully reviewing this,” one official told The Daily Beast.

McCain has long been a diehard supporter of Petraeus (he has called him one of America’s “greatest generals”), so his defense now is not surprising. But the senator may also have mounted an effective legal argument against demoting Petraeus, one that the retired general himself could use.

One needn’t even look to administrative law, as McCain did. The Army’s own regulations state that a rank determination can be re-opened after an officer has left military service “if substantial new evidence discovered contemporaneously with or within a short time following separation could result in a lower grade determination.”

In other words, the Pentagon may have waited too long to try to demote Petraeus.

Petraeus committed his offense in 2011 while he was still in uniform, but his actions weren’t discovered until more than a year after he left military service. And, legal experts said, Petraeus had good reason to believe that when he reached a plea agreement with federal prosecutors in 2015, the matter of demotion was behind him. A judge fined Petraeus $100,000 and gave him two years probation.

“No reasonable observer would consider this four-plus years period as a short time,” Eugene Fidell, who teaches military justice at Yale Law School, told The Daily Beast. “The regulation binds the government, and as I read it, it’s too late to reopen Gen. Petraeus’s” rank determination.

Another military law expert reached a similar conclusion.

“Reopening a matter that has already been settled—and decided in General Petraeus’ favor—sends a harmful signal throughout the ranks of senior uniformed leaders,” Butch Bracknell, a retired Marine officer and military legal adviser, wrote on Lawfare. Bracknell suggested that in light of Petraeus’s long record of otherwise unblemished service, Carter should consider “giving the man a break.”

“Certainly, there needs to be accountability for serious misconduct at all ranks of military service,” Bracknell argued. “But the unintended message of this ex post facto reduction in grade would be to tell junior generals and colonels that no matter how well you serve, no matter how much you sacrifice in service to our nation, there is always peril that senior civilian leaders will throw you under a bus for even a minor indiscretion—and that jeopardy never ends.”

Some of Petraeus’s fellow soldiers aren’t so sure. A number of officers still in uniform privately told The Daily Beast that dredging up the case so many years after the fact was gratuitous, especially for a commander who arguably was one of his generation’s best wartime commanders. Many credit Petraeus’s leadership in Iraq in 2007 and 2008 with stemming years of increasingly sectarian violence, eventually routing the precursor to ISIS.

Others officers, particularly lower ranking ones, said that the Defense Department must treat Petraeus like any other officer. Many said they know of comrades whose careers ended over far less serious infractions than sharing highly classified information with a civilian.

“I would have been kicked out of the Army for that,” one officer told The Daily Beast.

Others said that if they had shared classified information, they would certainly lose rank. The rules, they argued, should apply throughout the chain of command.

Pentagon officials said that while they understand McCain’s concerns, they are legally allowed to review Petraeus’s four-star standing.

“It’s certainly a reasonable line of questioning,” one defense official said of McCain’s comments. “But there is nothing the department is doing that is illegal. It might be distasteful to some but that is what comes with being a senior ranking officer.”

—with additional reporting by Tim Mak