The judge overseeing Donald J. Trump’s criminal trial in Manhattan rebuked the former president on Monday for mounting “a direct attack on the rule of law,” holding him in contempt of court for a second time and threatening to jail him if he continued to break a gag order that bars him from attacking jurors.

In a moment of remarkable courtroom drama, the judge, Juan M. Merchan, addressed Mr. Trump personally from the bench, saying that if there were further violations, he might bypass financial penalties and place the former president behind bars.

Justice Merchan acknowledged that jailing Mr. Trump was “the last thing” he wanted to do, but explained that it was his responsibility to “protect the dignity of the justice system.”

The judge said that he understood “the magnitude of such a decision” and that jailing Mr. Trump would be a last resort. He noted: “You are the former president of the United States, and possibly the next president as well.”

As the judge delivered his admonition and imposed a $1,000 fine, Mr. Trump stared straight at him, blinking but not reacting, and when the remarks were over, the former president shook his head.

It was the second time in two weeks that Mr. Trump had been punished for breaking the gag order, which also bars him from attacking prosecutors, witnesses and others. Among the violations of which Mr. Trump has been accused, Justice Merchan has taken those involving the jury most seriously.