Mr. Trump’s interest in directing law enforcement decisions extends beyond his political opposition but carries its own risk. The president’s support for capital punishment for the New York terrorism suspect, Sayfullo Saipov — “SHOULD GET DEATH PENALTY,” he tweeted — could pose problems for prosecutors and help defense lawyers who could argue that their client cannot get a fair trial.

Mr. Trump also weighed in again on Friday on the case of Sgt. Bowe Bergdahl, who pleaded guilty to desertion and endangering other troops by walking away from his base in Afghanistan and getting captured by the Taliban. Mr. Trump, who last year called Sergeant Bergdahl a “dirty rotten traitor” who should be executed, expressed outrage when a military judge on Friday gave the sergeant a dishonorable discharge but no jail time.

“The decision on Sergeant Bergdahl is a complete and total disgrace to our Country and to our Military,” Mr. Trump tweeted.

But Mr. Trump’s own outspokenness may have helped lead to the very result he was condemning. The judge did not explain his reasoning on Friday but last week said he would consider the president’s past comments as evidence for a lighter sentence.