On a sunny day at the Oasis Diner in Plainfield, Indiana, roughly 20 miles west of Indianapolis, Ted Cruz breezed through a 1950s-era diner with a gleaming steel facade. One of the last diners of its kind, the Oasis sits along the Historic National Road, the iconic highway traversing the state that fancies itself the Crossroads of America. Here, four days before he named Carly Fiorina as his vice presidential pick, Cruz met a crowd of a few hundred supporters. He mugged for photos, noshed on a fried pickle offered to him by a customer, then shot straight behind the...