The number of hate groups in the United States rose for the fourth year in a row in 2018, pushed to a record high by a toxic combination of political polarization, anti-immigrant sentiment and technologies that help spread propaganda online, the Southern Poverty Law Center said Wednesday. The number of hate groups rose by 7 percent last year to 1,020, a 30 percent jump from 2014. That broadly echoes other worrying developments, including a 30 percent increase in the number of hate crimes reported to the F.B.I. from 2015 through 2017 and a surge of right-wing violence that the Anti-Defamation League said had killed at least 50 people in 2018. The center's findings run parallel to a report on extremist-related killings in the United States that was issued last month by the Anti-Defamation League's Center on Extremism. That report said that right-wing extremism was linked to every extremist-related killing the group tracked in 2018, at least 50, and that jihadist groups were linked to none. It said that made 2018 the deadliest year for right-wing extremism since the 1995 Oklahoma City bombing. The law center and the Anti-Defamation League both pointed to the killing of 11 people at the Tree of Life Synagogue in Pittsburgh in October as a symptom of the increasingly combustible mix of anti-immigrant sentiment, violence and online conspiracy-mongering.