Honey, switch to the 10-pounders,” says Kirsten Gillibrand. This directive is made in a dreary, windowless gym on the fourth floor of a Courtyard by Marriott hotel in Waterloo, Iowa. At 7:15 a.m. the Sunday before Memorial Day, press secretary Alex Phillips and I are blearily sweating through a Gillibrand-devised circuit the senator correctly predicts will leave us sore tomorrow. She’s using 25-pound barbells for the exercise that she wants me to use 10 pounds for. She’s encouraging. Nurturing. Appropriately bossy. As she corrects my foot position during triceps kickbacks, I feel the blessed relief of surrendering to competence. I’d asked to attend the workout of the senator from New York and aspiring president after seeing her do chest presses on Instagram, thinking it would work as a facile metaphor for the strength she’d need to break out in a 24-person Democratic field. I’d hoped the sight of 52-year-old Gillibrand’s now-famous biceps might reveal some larger, heretofore obscured appeal. Some reserve of magnetism, also hiding under a navy blazer. A glimpse into the reasons she’s not gaining ground as a candidate. The majority of Democratic hopefuls have yet to experience a ...