On a balmy night in Austin, against the backdrop of the glittering high-rises of the city’s booming downtown, Beto O’Rourke is laying out his audacious plan to change the face of Texas, and America. In front of him, packed into an open-air park, a largely young crowd of 40,000 is thrumming with scarcely contained glee. Even in the liberal bubble of Austin they have never experienced anything like this: a Democrat seriously in the running for a Senate seat, vying to topple Ted Cruz, the Tea Party fanatic whom even fellow Republicans call “Lucifer in the flesh”. “We are not...