Ms. James, in her first public appearance since her federal indictment, said that she and Mr. Mamdani knew what it was like to be threatened and harassed.

The president’s bellicose vow of steep new tariffs, followed quickly by a more conciliatory message, pointed to an internal tug of war over his approach.

Representative Mikie Sherrill, a New Jersey Democrat, said a firm once owned by her Republican opponent, Jack Ciattarelli, spread misinformation about opioids. His campaign has threatened to sue her for defamation.