Trump's comment came as he described the violence in inner-city neighborhoods as "far more dangerous than Afghanistan." Trump said that "Clinton lives behind gates and walls and guards" and that working-class families "just want a fraction of the security enjoyed by our politicians and certainly enjoyed by her."
Trump made a similar comment in May when he addressed a National Rifle Association conference. Trump tweeted on May 21: “Crooked Hillary wants to get rid of all guns and yet she is surrounded by bodyguards who are fully armed. No more guns to protect Hillary!”
Trump argued, as he often does, that if the innocent people at the sites of recent mass shootings had been armed, the outcomes would have been different. The remarks were a version of a pro-gun argument that the country would be safer if more people were armed.
But the insinuation of gun-related violence against a rival is unprecedented in modern presidential politics. Trump also routinely claims that Clinton wants to ban all guns or repeal the Second Amendment, which is false.
In a statement, Clinton campaign manager Robby Mook said, "Donald Trump has a pattern of inciting people to violence. Whether this is done to provoke protesters at a rally or casually or even as a joke, it is an unacceptable quality in anyone seeking the job of commander in chief."
After doubting Clinton's stamina and ability to stand and campaign in a hot, crowded room earlier this week, Trump lobbed similar attacks on Friday, saying the president needs to balance different responsibilities.
"To do these things, you need a lot of energy — she doesn't have the energy," Trump said. "And, in my opinion, she is totally unfit to be the president of the United States."
At another point, Trump bragged that he often does multiple campaign events a day. "Do you think Hillary Clinton can get through one?" he asked the crowd.
Clinton recently rejoined the campaign trail after taking some time off. She fell ill at a 9/11 ceremony on Sunday. Only afterward did her campaign say that she had recently received a diagnosis of pneumonia.
At the rally, Trump took on other criticisms he has faced for using anti-Muslim, anti-immigrant rhetoric. He said that when Democrats "are in trouble they always pull out the 'racist' word." He said that "people who warn about radical Islamic terrorism are not Islamophobes."
Trump faced protesters inside and outside his rally. He also unveiled a new element to his introduction, taking the stage to music from "Les Miserables" against a backdrop reading: "Les Deplorables." The sign was a reference to Clinton's labeling half of his supporters a "basket of deplorables," a comment she later said she regretted.
Trump also pledged to undo President Obama's executive actions opening up relations with Cuba, "unless the Castro regime meets our demands."
After Trump concluded his speech, his campaign sent out a news release with his remarks, highlighting his comments about Cuba and Venezuela — but misspelling the latter "Venezuala."
The prepared remarks from the campaign did not include the line "Let's see what happens to her."
Johnson reported from Washington.