If it was a joke, nobody laughed very loudly.

In comments that once again placed him at the centre of fresh controversy, Donald Trump appeared on Tuesday to suggest that if Hillary Clinton was elected president, the only way to stop her choosing liberal Supreme Court justices would be to shoot her.

“Hillary wants to abolish, essentially abolish the Second Amendment,” he said, referring to the piece of legislation that gun rights activists claim gives them the right to bear weapons. “If she gets to pick her judges, nothing you can do, folks. Although the second amendment people, maybe there is.”

Mr Trump was speaking in North Carolina (AP)

Mr Trump’s comment came during a rally in Wilmington, Delaware, and was immediately seized on by commentators as the latest example of the New York tycoon’s offhand language, and willingness to step beyond well beyond what would be considered normal during an election campaign.

As Mr Trump and his supporters sought to defuse the incident and insist he was merely talking about gun rights activists voting for him rather than Ms Clinotn, the Secret Service, which is responsible for the security of the president - as well as the two presidentiall candidates - said it was "aware" of the tycoon's comments.

Two weeks ago the Republican candidate sparked huge controversy when he called on Russia to hack into Ms Clinton’s emails. The following day, amid outcry from people across the political spectrum, Mr Trump said he was simply joking.

Donald Trump's most controversial quotes Show all 14 1 /14 Donald Trump's most controversial quotes Donald Trump's most controversial quotes On Isis: "Some of the candidates, they went in and didn’t know the air conditioner didn’t work and sweated like dogs, and they didn’t know the room was too big because they didn’t have anybody there. How are they going to beat ISIS?" Getty Donald Trump's most controversial quotes On immigration: "I will build a great wall — and nobody builds walls better than me, believe me —and I’ll build them very inexpensively. I will build a great, great wall on our southern border, and I will make Mexico pay for that wall. Mark my words." Reuters Donald Trump's most controversial quotes On Free Trade: "Free trade is terrible. Free trade can be wonderful if you have smart people. But we have stupid people." PAUL J. RICHARDS | AFP | Getty Images Donald Trump's most controversial quotes On Mexicans: "When Mexico sends its people, they’re not sending their best. They’re sending people that have lots of problems. They’re bringing drugs. They’re bringing crime. They’re rapists." Getty Donald Trump's most controversial quotes On China: "I just sold an apartment for $15 million to somebody from China. Am I supposed to dislike them?... I love China. The biggest bank in the world is from China. You know where their United States headquarters is located? In this building, in Trump Tower." Getty Images Donald Trump's most controversial quotes On work: "If you're interested in 'balancing' work and pleasure, stop trying to balance them. Instead make your work more pleasurable." AP Donald Trump's most controversial quotes On success: "What separates the winners from the losers is how a person reacts to each new twist of fate." Donald Trump's most controversial quotes On life: "Everything in life is luck." AFP Donald Trump's most controversial quotes On ambition: "You have to think anyway, so why not think big?" Getty Images Donald Trump's most controversial quotes On his opponents: "Bush is totally in favour of Common Core. I don't see how he can possibly get the nomination. He's weak on immigration. He's in favour of Common Core. How the hell can you vote for this guy? You just can't do it." Reuters Donald Trump's most controversial quotes On Obamacare: "You have to be hit by a tractor, literally, a tractor, to use it, because the deductibles are so high. It's virtually useless. And remember the $5 billion web site?... I have so many web sites, I have them all over the place. I hire people, they do a web site. It costs me $3." Getty Images Donald Trump's most controversial quotes On Barack Obama: "Obama is going to be out playing golf. He might be on one of my courses. I would invite him. I have the best courses in the world. I have one right next to the White House." PA Donald Trump's most controversial quotes On himself: "Love him or hate him, Trump is a man who is certain about what he wants and sets out to get it, no holds barred. Women find his power almost as much of a turn-on as his money." Getty Images Donald Trump's most controversial quotes On America: "The American Dream is dead. But if I get elected president I will bring it back bigger and better and stronger than ever before and we will make America great again." GETTY

On, Tuesday after, Ms Clinton's responded to Mr Trump's comments, saying it was proof of his unsuitability for the office. "This is simple - what Trump said was dangerous. A person seeking to the president of the US should not be suggesting violence in any way," said a spokesperson.

The comments came one day after 50 senior Republican national officials published a public letter in which they said Mr Trump was inappropriate to occcupy the White House.

“From a foreign policy perspective, Donald Trump is not qualified to be president and commander-in-chief,” said the letter.

“Indeed, we are convinced he would be a dangerous president.”

Among those who signed the high profile people to sign the letter was Gen Michael Hayden, former director of CIA and National Security Agency. On Tuesday, Mr Hayden said on CNN he was startled by Mr Trump's latest comment.

He said it was either a "very bad taste" joke about political assassination, or simply an indication of Mr Trump's insensitivity to the prevalanance of assassination in American politics, "and that is something we never come close to".

Mr Trump's campaign later rejected that the tycoon was doing anything more than calling for supporters of the Second Amendment to rally around him.

“It’s called the power of unification – 2nd Amendment people have amazing spirit and are tremendously unified, which gives them great political power,” Jason Miller, the campaign’s senior communications adviser, said in a statement.