The number of active hate groups in the United States has surged to a 20-year high, according to a report released Wednesday by the Southern Poverty Law Center, which claims that younger extremists prefer “polo shirts and khakis to Klan robes.” The groups include neo-Nazis, white supremacists, black nationalists and neo-confederates, among others. The civil-rights organization counted 1,020 hate groups in its annual survey, up from 784 just four years earlier. The most significant recent growth has occurred in white nationalist organizations, up from 193 to 264 in the past two years, said Heidi Beirich, director of SPLC’s Intelligence Project.