To get herself ready, she has studied Mrs. Clinton’s policy positions, watched videotapes of her in speeches and debates, and gotten into character by wearing a “bright blue blazer” for a session in October. Mimicking Mrs. Clinton’s good posture and poised stage presence is critical to preparing her boss to face one of the 2016 campaign’s most polished debaters.

Ms. Crowell can also imitate Mrs. Clinton’s sometimes folksy delivery, as she did in an interview on Saturday: “If that’s his idea of a revolution, friends, I’m worried. I’m very worried.”

Noting that the senator “is a mover,” the faux Mrs. Clinton said that if Mr. Sanders ever got too close with his hands at future rehearsals, she had a rebuke ready. “I’ll mention that maybe he could keep his hands more in his personal space,” she said.

Mr. Sanders begrudgingly came to debate preparations. (“It would be better if you were able to practice someone getting under your skin before they get under your skin,” Ms. Crowell said she had told him). She said she relished the opportunity to turn the tables on a boss whom she described as often “critiquing” his staff members. Plus, it gives her the chance to “pretend to be a presidential candidate.”