“Did you see that? I mean, my lord. What have we become if we abide by this? So much we can do and must do,” Mr. Biden said.
Even some congressional Republicans questioned the post.
Senator Mitt Romney told reporters on Tuesday he thought the tweet was “shocking.” Senator John Thune, the No. 2 Senate Republican, said, “It’s a serious accusation, which should only be made with facts and evidence, and I haven’t seen any yet.”
At the end of May, Mr. Trump announced — again on Twitter — that he intended to designate Antifa as “a terrorist organization.” There were, however, two problems with the declaration: The federal government can only designate foreign-based groups as terrorist entities, and anti-fascism is a political idea, like pacifism or communism, not an organization.
With tens of thousands of people marching across the country, it is all but impossible to accurately determine how many Antifa activists have taken part in the protests. But it is definitely clear that they are not playing a leading role in the protests.
Terrence Bisson, a mathematics professor who has known Mr. Gugino for a decade, mostly through the Western New York Peace Center, said his friend would remain in the hospital for weeks. Mr. Gugino was still in a delicate condition, disturbed by bright lights and unable to move his head without tremendous pain, he said.
Sage Green, a former program manager at PUSH Buffalo, a local activist group, said the last time she saw Mr. Gugino was early in the spring when he was asked to critique the presentations of some students in an environmental studies class at the University at Buffalo.
“He was there giving feedback almost in a grandfatherly way,” Ms. Green recalled. “He was telling them they were all doing great work.”