It’s the fixation of Washington. Now Democrats from coast to coast are plotting how to capitalize on the Jeffrey Epstein saga. An ongoing furor around the government’s investigation of the disgraced financier has done more than fracture President Donald Trump’s base. It has also injected a volatile new element into midterm elections and created an opening for Democrats scrambling to exploit any perceived Republican weakness as they work to retake the House. Once largely confined to the Beltway and to the more conspiratorial corners of the internet, the Epstein affair has already shown surprising staying power by tapping into pervasive...