A federal statute defines domestic terrorism but carries no penalties. First and Second Amendment concerns make prosecuting these cases difficult.

According to the authorities, Mr. Lemley and Mr. Mathews made a functioning assault rifle. They also bought more than 1,500 rounds of rifle ammunition, fired the rifle at a Maryland gun range and acquired vests to hold body armor. Although the charges were not directly linked to the Richmond rally, law enforcement officials said the three men had discussed attending it.

Additionally, three other men associated with the Base being tracked by the F.B.I. were arrested on Thursday by the authorities in Georgia. Officials said they were plotting to kill a couple belonging to the loose affiliation of radical activists known as antifa.

Adherents of extremist groups have been beating the drums for people to participate.

One online meme shows half a dozen men who carried out bloody attacks in the United States, Norway and New Zealand, dressing them as biblical saints with halos above their heads. “Virginia Is For” read the headline.

Many of the comments are racist, anti-Semitic and unprintable. “Y’all need to go full white ethnostate and really set the pace for 2020,” said another online message, below the picture of a road sign that had been altered to read “Virginia Is For Gun Lovers.”

The call to arms by others prompted fears that the Richmond rally would foreshadow the tenor of political events throughout what promises to be a fraught presidential election.

“This is about those who want to co-opt these moments and turn it into the start of a civil war or some sort of race war,” said Oren Segal, the director of the Anti-Defamation League’s Center on Extremism. “A lot of this is hyperbole, but who at this point would take that lightly.”