When an alarmed Representative Kevin McCarthy, the minority leader, called the White House on Jan. 6, 2021, demanding to know why the president of the United States had suggested he was coming to the Capitol while Congress met to certify his election defeat, the person on the other end of the line had just turned 25 years old.

“I said, ‘I’ll run the traps on this,’” Cassidy Hutchinson, now 26, testified this week before the House select committee investigating the Jan. 6 attack, recalling what she had told Mr. McCarthy, Republican of California. “I can assure you, we’re not coming to the Capitol.”