The United States has always been plagued by violence associated with white nationalism. Violence associated with white nationalism has spiked at intervals in recent decades, including the Oklahoma City bombing in 1995. But government data shows that, since the Sept. 11, 2001, terrorist attacks, far-right extremists carried out nearly three times as many attacks on Americans in the United States as Islamic extremists did.

Incidents of white supremacist propaganda such as the posting of fliers increased 182 percent in 2018, from 421 in 2017 to 1,187 last year, and the number of racist rallies increased nearly 20 percent, according to the Anti-Defamation League. Hate crimes have increased over the past several years, and reached a nine-year high in 2017, the latest year available, according to F.B.I. data.

“Individuals adhering to racially motivated violent extremism ideology have been responsible for the most lethal incidents among domestic terrorists in recent years,” Michael C. McGarrity, an official in the F.B.I.’s counterterrorism division, testified before Congress in June.

The figures were already rising at the time Mr. Obama won the presidency. But the backlash against the 2009 report from the Homeland Security Department underscored how the nexus of race, national security and civil liberties would limit the effort to confront the problem.

Former law enforcement officials say white supremacists were energized by the 2008 election. On social media they discussed the possibility of a race war should Mr. Obama become president. Hate crimes peaked that year in October, according to the National Archive of Criminal Justice Data, when his election seemed assured.

And traffic to white nationalist websites like Stormfront increased after the election, said Derek Black, a former white supremacist whose father founded Stormfront. Mr. Black, who has left the movement, said that having a black president motivated new recruits to join.

The Homeland Security Department report, issued amid this activity, predicted the growth of “right-wing extremist activity, specifically the white supremacist and militia movements,” and said the recession could help recruitment because people economically at risk were more susceptible to extremists of all stripes. Internet platforms, the report said, would let domestic extremists meet and radicalize others individuals — just as foreign extremists like the Islamic State would do.