A few dozen influential Democratic donors in Los Angeles settled into into a plush gold-and-cream-accented living room in Holmby Hills for what was to be a cozy debate night, happy to be hosted by James Costos, a former HBO executive and Barack Obama’s ambassador to Spain, and his husband, the celebrity interior designer Michael Smith. They were joined by Second Gentleman Doug Emhoff and a trio of governors who figured to be a big part of Democrats’ post-2024 future...