In recent weeks, Vice President Kamala Harris has dashed off to Florida on short notice. She sparred with the state’s conservative governor, Ron DeSantis, over how to teach slavery in schools. And she flew into Iowa to defend abortion rights while 13 Republican presidential candidates were having dinner a few miles away. Although her words were directed at Republicans, her message was also aimed at all her doubters. Once a rising star as a senator in California, Ms. Harris has for years been saddled by criticism of her performance as vice president. She has struggled with difficult assignments on issues...