Former CIA Director Michael Hayden, who opposes Trump, called Trump's comment showed "an incredible insensitivity to the prevalence of a political assassination inside of American history."
Sen. Chris Murphy of Connecticut warned Trump that "unstable people with powerful guns and an unhinged hatred for Hillary are listening to you."
Four U.S. presidents have been killed by gunshot: Abraham Lincoln, James Garfield, William McKinley and John F. Kennedy. In 1968, Democratic presidential hopeful Robert Kennedy was shot and killed. In 1981, President Ronald Reagan survived an attempted assassination after he was shot in the chest.
A pattern of aggressive moments on the campaign trail colors the debate over what Trump meant. Many Trump rallies have erupted in physical violence, with protesters being sucker punched, minorities being taunted, and crowds regularly chanting "Lock her up!" at the mere mention of Clinton's name. Trump himself has embraced violent language as well, at one point in February saying that he'd like to punch a heckler in the face.
Last week, in an interview with Breitbart News, longtime Trump ally Roger Stone warned of "rigged" election that would result in a "bloodbath" if Trump was not victorious. "If you can't have an honest election, nothing else counts," Stone said. "I think he's gotta put them on notice that their inauguration will be a rhetorical, and when I mean civil disobedience, not violence, but it will be a bloodbath… We will not stand for it."
In the past, Stone has called for politicians like Clinton and Bernie Sanders to be executed for purported crimes ranging from "treason" to "murder."
In a Tuesday night interview on Fox News' "Hannity," Trump brushed aside the controversy.
"The NRA, as you know, endorsed me — they're terrific people," Trump said. "Wayne [LaPierre] and Chris [Cox] and all the people over there and they tweeted out, basically they agree 100 percent with what I said. And there can be no other interpretation. Even reporters have told me. I mean, give me a break."
Dave Carney, a New Hampshire-based GOP strategist who advised Newt Gingrich and Rick Perry during their presidential bids, agreed with the nominee. "He is not a polished politico," Carney said. "That's part of his strength and it both endears him to his supporters and it infuriates his opponents who love to play the victim card every chance they get."
Though controversial comments like these have limited Trump's ability to appeal to undecided voters, his core supporters didn't seem fazed by it, and many accused the media of twisting his words.
"Harmless!" said Texas millionaire Doug Deason, who is giving the maximum contribution allowed to Trump and helping raise money from others. "The left, which means the entire press corps, will twist everything he says from now until the election. Won't hurt the race at all."
"Outrageous media bias," Stone said Tuesday, calling it "an attempt to 'Goldwaterize' him," a reference to 1964 Republican nominee Barry Goldwater, who lost in a landslide.
But Trump's latest controversial remarks came one day after Trump sought to move past a stretch of controversy with a speech aimed at unifying the Republican Party. Trump couldn't go a day without shooting himself in the foot, said David Axelrod, a former top adviser to President Barack Obama.
"I don't think he thought it through. It was a red meat line he tossed off carelessly to signify solidarity with the crowd," Axelrod said. "But that is the problem: When you are the president of the United States you can't do that. The things you say can send armies marching and markets tumbling. And he seems incapable of controlling himself. This is at the core of worries about him."
Vice presidential candidate Mike Pence, speaking to a few hundred supporters later Tuesday in Pittsburgh, didn't acknowledge the flap as he reinforced the main points of Trump's statements.
The rally, in a battleground state where a poll today showed Clinton widening a lead, saw Pence playing a role he's assumed in the campaign: saying what Trump meant to say in a non-controversial way and without really acknowledging the underlying controversy.
"We are not just choosing a president, we are choosing in that president a direction for the rule of law in this country," Pence said referring to judicial appointments he said could impact abortion rights and the Second Amendment."
Pence said each day the national press "latches on" to another Trump comment and tries to sink him. He pointed to the attendance by the father of the Orlando terrorist at a Hillary Clinton rally, and his comment that he supports Clinton in part because she's good on national security.
"The media stays focused on our side of the aisle not theirs," Pence said. "It's 2-on-1 with the media, who are doing most of Hillary Clinton's work for her but Donald Trump is still winning with the American people."