Trump alarmed many in the political world and beyond on Tuesday when he suggested that "Second Amendment people" -- those who support gun rights -- could act against his Democratic rival Hillary Clinton or the justices she would appoint to the US Supreme Court if she were president.
It was beyond the pale for the Kennedys, who wrote in a Washington Post op-ed that Trump's "dark and offensive rhetoric" should disqualify him from the presidency.
"Today, almost 50 years later, words still matter," the Kennedys wrote.
"So it was with a real sense of sadness and revulsion that we listened to (Trump) as he referred to the options available to 'Second Amendment people,' a remark widely, and we believe correctly, interpreted as a thinly veiled reference or 'joke' about the possibility of political assassination."
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"As the daughter of a leader who was assassinated, I find Trump's comments distasteful, disturbing, dangerous," Bernice King posted on Twitter late Tuesday.
King's murder in April 1968 triggered a wave of social unrest that included riots in several US cities.
Reagan's daughter Patti Davis posted a scathing open letter to Trump on her Facebook page, saying that "your glib and horrifying comment about 'Second Amendment people' was heard around the world."
"It was heard by the person sitting alone in a room, locked in his own dark fantasies, who sees unbridled violence as a way to make his mark in the world, and is just looking for ideas," Davis added.