In early 2020, Representative Alexandria Ocasio-Cortez, the progressive Democrat from New York, was asked to speculate about her role under a Joe Biden presidency. She groaned. “In any other country, Joe Biden and I would not be in the same party,” she said, “but in America, we are.” Ms. Ocasio-Cortez’s frustration with the two-party system reflects the frustration American voters feel every time they step into the voting booth, when they find themselves stuck with the same two choices — and, in most places, only one with any shot at winning. As a new Congress sputters into gear, this rusty...