“I appreciate President Trump taking the gloves off when it comes to unleashing our military on ISIL,” Mr. Graham said, referring to the battlefield gains against the Islamic State in Iraq and Syria. “However, I’m dumbfounded as to why the Trump administration still follows the Obama playbook when it comes to dealing with terror suspects.”
That drew a retort from Vice President Mike Pence. “I got to tell you, there’s nothing about President Trump’s policies that mirror the last administration,” he told Brian Kilmeade on Fox News Radio.
Skeptics of the military system were relieved by Mr. Trump’s turnabout. “It’s good that President Trump has realized that a prosecution of Saipov in federal court would be much faster and less legally complicated than sending him to Guantánamo for possible trial before military commission,” said John B. Bellinger III, a top White House and State Department lawyer under President George W. Bush.
While Mr. Trump opted for a civilian handling of Mr. Saipov, the president’s call for capital punishment introduced a surprise complication that may burden prosecutors and help defense lawyers. Mr. Trump first broached the subject in a Twitter message posted shortly before midnight on Wednesday evening.
Presidents are typically advised never to weigh in publicly on pending criminal cases. Such comments can be used by defense lawyers to argue that their clients cannot get a fair trial — especially when the head of the executive branch that will prosecute a case advocates the ultimate punishment before a judge has heard a shred of evidence at trial.
But Mr. Trump is not one for cautious detachment, and he has disregarded such advice before. Just this week, a military judge said he would consider similar comments by Mr. Trump as evidence in favor of a lighter sentence for Sgt. Bowe Bergdahl, who pleaded guilty to desertion and endangering fellow troops by walking away from his post in Afghanistan, where he was later captured and held prisoner by the Taliban for five years.
Lisa Monaco, who ran the Justice Department’s national security division under Mr. Obama, said presidential comments “violate the golden rule” of independent law enforcement, could taint jury pools and call into question the rigorous process used to decide whether to seek the death penalty.
“It doesn’t help the prosecutors,” she said. “It undermines the ability of the career prosecutors to do their job.”
Other presidents have been criticized for offering public verdicts about pending criminal cases. In 1970, President Richard M. Nixon declared that Charles Manson “was guilty, directly or indirectly, of eight murders without reason” in the middle of his trial in the killings of the actress Sharon Tate and others.
By the end of the day, the Manson team’s lawyers had moved for a mistrial, citing the president’s remarks, and Nixon issued what his press secretary called a “clarification” taking them back.
“The last thing I would do is prejudice the legal rights of any person, in any circumstances,” Nixon said. The defendant later held up in court a newspaper with the headline “Manson Guilty, Nixon Declares.” But the judge allowed the trial to proceed, ultimately ending with a conviction.
In 2005, Mr. Bush expressed confidence that Representative Tom DeLay of Texas, the former Republican majority leader, would be acquitted, weeks before his trial on money laundering charges was to open. “I hope that he will, because I like him, and plus, when he’s over there, we get our votes through the House,” Mr. Bush told a television interviewer.
His successor, Mr. Obama, forecast an execution for Mr. Mohammed, the Sept. 11 detainee. Defending the later-aborted decision to try Mr. Mohammed in civilian court rather than a military tribunal, Mr. Obama said critics would not find it “offensive at all when he’s convicted and when the death penalty is applied to him.”
The impact of such comments is more pronounced in military justice cases since the president is the commander in chief of the judges and juries that determine guilt or innocence and hand down sentences.
Responding to a wave of sexual harassment allegations in the military, Mr. Obama declared in 2013 that troops who commit sexual assault should be “prosecuted, stripped of their positions, court-martialed, fired, dishonorably discharged.” In this instance, he was not commenting on a particular defendant, but lawyers nonetheless argued that it constituted “unlawful command influence.”
Such influence refers to actions by commanders that could be seen as an attempt to sway a court-martial. Defense lawyers in multiple cases cited Mr. Obama’s words. In one case in South Carolina, a judge noted the command influence issue in dismissing sexual assault charges against an Army officer. In another in Hawaii, a Navy judge decided that two defendants could not be punitively discharged because of the president’s comments.
Mr. Trump has more than once offered strong words about people suspected of major crimes. He called the man who opened fire on a concert in Las Vegas last month a “very sick man” and “a very demented person,” but since the man killed himself, there will be no trial to influence. On Wednesday, he called Mr. Saipov “this animal.”
As a candidate, Mr. Trump was unflinchingly vocal about Sergeant Bergdahl, calling him a “dirty rotten traitor” who should be executed. A military judge in February called the comments “disturbing and disappointing,” but decided that since they were made when Mr. Trump was a private citizen, not the president, they did not constitute undue command influence.
Mr. Trump was more restrained when asked about Sergeant Bergdahl’s case last month, but not so much that it did not come up in court. “I can’t comment on Bowe Bergdahl,” Mr. Trump told reporters in the Rose Garden. “But I think people have heard my comments in the past.”
After concerns were raised about the “but” in his comment, the White House sought to mitigate any possible damage with a statement. “The president expects all military personnel who are involved in any way in the military justice process to exercise their independent professional judgment, consistent with applicable laws and regulations,” the statement said.
Col. Jeffery R. Nance, the Army judge presiding over the case, rejected a request to dismiss the case or limit the potential sentence because of Mr. Trump’s remarks, saying he had not been influenced. But he indicated that he would weigh the president’s comments before determining punishment. “I will consider the president’s comments as mitigation evidence as I arrive at an appropriate sentence,” he said.
Mr. Bellinger, the former Bush White House lawyer, noted that the Bergdahl case had already resulted in one White House statement walking back the president’s words. “The White House,” he said, “may now need to issue a similar statement with respect to a federal prosecution of Saipov.”