Eight years ago, just before the Iowa caucuses, Donald J. Trump crowed about his invulnerability.

“I could stand in the middle of Fifth Avenue and shoot somebody, and I wouldn’t lose any voters, OK?” he said. “It’s, like, incredible.”

On Tuesday, at a federal appeals court argument held the week before this year’s caucuses, a lawyer for Mr. Trump said that the Constitution basically states the same thing.

It took a few questions from Judge Florence Y. Pan to pin down the lawyer, D. John Sauer. But in the end he made the jaw-dropping claim that former presidents are absolutely immune from prosecution even for murders they ordered while in office.

“I asked you a yes-or-no question,” Judge Pan said. “Could a president who ordered SEAL Team 6 to assassinate a political rival, who was not impeached, would he be subject to criminal prosecution?”