Members of the Kennedy family sternly rebuked Donald Trump today for joking about "the possibility of political assassination," joining the relatives of other slain or wounded US leaders in condemning recent rhetoric by the Republican presidential nominee.

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.

"Political violence is the greatest of all civic sins," wrote William Kennedy Smith and Jean Kennedy Smith, nephew and sister of Democratic president John F Kennedy, who was assassinated in 1963, and senator Robert Kennedy, who was assassinated in 1968 as he campaigned for president.

"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." 

The Kennedy column followed criticism by the daughter of civil rights leader Martin Luther King Jr., who was slain two months before Robert Kennedy, and the daughter of Republican Ronald Reagan, who was wounded by a would-be assassin in 1981, 69 days into his presidency.

"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." 

The remark was heard by "sane and decent people who shudder at your fondness for verbal violence," as well as by Trump supporters who angrily yell, "Lock her up" about Hillary Clinton.

"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.