By articulating their audiences’ hatred, both men effectively encouraged them to act on it.

Mr. Wallace’s rallies regularly erupted in violence, as his fans often took his words not just seriously but also literally. Mr. Wallace often talked about dragging hippies “by the hair of their head.” At a Detroit rally in 1968, his supporters did just that, dragging leftist protesters out of their seats and through a thicket of metal chairs. As they were roughed up, the candidate signaled his approval from the stage: “You came here for trouble and you got it.”

Mr. Trump’s rallies have likewise been marked by violence unseen in other modern campaigns. At a 2015 rally in Birmingham, Ala., for example, an African-American protester was punched, kicked and choked. Rather than seeking to reduce the violence from his supporters, Mr. Trump rationalized it, saying “maybe he should have been roughed up, because it was absolutely disgusting what he was doing.”

This leads us to the significant difference between Mr. Wallace and Mr. Trump. Mr. Wallace’s targets were, for the most part, presented in the abstract. Though he denounced broad categories of generic enemies — “agitators,” “anarchists” and “communists” — he rarely went after an individual by name.

Mr. Trump, in pointed contrast, has used his rallies to single out specific enemies. During the 2016 campaign, he demonized his political opponents in the primaries and the general election, and also denounced private individuals, from Megyn Kelly, the former Fox News anchor, to the former Miss Universe Alicia Machado and the federal judge Gonzalo Curiel.

At recent rallies, he has targeted four Democratic House members who have criticized him and his administration — Representatives Ilhan Omar, Alexandria Ocasio-Cortez, Rashida Tlaib and Ayanna Pressley.

Participants at Mr. Trump’s rallies have been moved to attack individuals he’s singled out. For most rally participants, the attacks have been confined to ominous but nevertheless nonviolent chants — from the 2016 cries of “Lock her up!” to the recent refrain of “Send her back!” But a handful have gone further, targeting the individuals named by the president with death threats and even attempts at violence.