Spencer Pratt was a reality TV villain. Now he wants to be LA.’s savior.

LOS ANGELES—Spencer Pratt knows he doesn’t need to be universally liked. At an annual tennis invitational and white party at a billionaire’s Beverly Hills mansion last weekend, the reality TV villain and mayoral hopeful marveled at his potential path to victory. “It’s pretty incredible you can become the mayor with only 51% of people liking you. I’m like, ‘I can do that,’” Pratt told the crowd. The 42-year-old rose to millennial fame as the dark prince of “The Hills,” playing the love interest of his now-wife, Heidi Montag. He provoked conflict with fellow cast members and was seen by fans...