The refrain has been splashed across T-shirts and bumper stickers and beer koozies. It has thundered through arenas in Milwaukee and Indianapolis and Chicago and Glendale and Savannah. Crowds wait for Kamala Harris to say it like they might anticipate joining Taylor Swift on the bridge of “Cruel Summer.” “We are not going back!” they roar. “We are not going back!” Here’s the thing, though: The line wasn’t designed to be a catchphrase. President Joe Biden had notified Harris on a Sunday morning...