For Rachel Conner, the 2018 election season has been a moment of revelation. A 27-year-old social worker, Ms. Conner voted for Hillary Clinton in the 2016 primaries, spurning the more liberal Bernie Sanders, whom many of her peers backed. But Ms. Conner changed course in this year’s campaign for governor, after concluding that Democrats could only win with more daring messages on issues like public health and immigration. And so on a recent Wednesday, she enlisted two other young women to volunteer for Abdul El-Sayed, a 33-year-old advocate of single-payer health care running an uphill race in Michigan to become...
