Donald Trump. Photo: Reuters

Controversial US billionaire Donald Trump has finally gone too far for some fellow candidates for the Republican presidential nomination.

He was not criticised when he called Mexican immigrants rapists, murderers and drug dealers, but was blasted when he disparaged Senator John McCain’s Vietnam War record.

After dismissing McCain’s reputation as a war hero because he was captured in Vietnam and saying “I like people who weren’t captured”, Trump declared: “I will say what I want to say.”

He insisted he would stay in the Republican primary field, despite rivals who say he has now shown he does not merit the presidency.

Trump’s candidacy has proved to be a gift to Democrats, especially front-runner Hillary Clinton, as they line up for their own bids for the nomination.

While he has taken the top spot in early national polls among Republicans, his brash, some say outlandish, world view would seem to make him unelectable in a nationwide election.

For the extraordinarily crowded Republican field, Trump has become a big problem.

His declarations have struck a chord with the deeply conservative Republican voter base – the group that is most energised to turn out for primary elections that decide the party’s presidential candidate.

At the same time, his comments – especially about Mexicans – have further alienated the growing block of Hispanic voters Republicans need to capture the White House. What is more, Trump is dominating media coverage of the Republican field, denying attention to more mainstream candidates who are trying to be noticed in a field that now numbers 15.

As fellow Republicans finally stepped out to cri-ticise Trump, he showed no sign of remorse for saying McCain was a war hero only “because he was captured”. McCain spent five years in a North Vietnam prison beginning in 1967, often undergoing torture, after his jet fighter was shot down.

Trump lashed out at his Republican presidential rivals as “failed politicians” and said he did not need “to be lectured by any of them”. But he appeared to tone down his rhetoric a little, saying: “If there was a misunderstanding, I would totally take that back. I don’t know if I made a mistake.”

McCain said Trump does not need to apologise to him for remarks about his long captivity in Vietnam, but should tell veterans and their families that he is sorry.

“When Trump says he prefers to be with people who are not captured, the great honour of my life was to be in the company of heroes,” McCain said, adding: “I am not a hero.”

“It’s not just absurd,” said Florida Senator Marco Rubio. “It’s offensive. It’s ridiculous. And I do think it is a disqualifier as commander-in-chief.”

Numerous other Republican candidates, including former Florida Governor Jeb Bush, former Texas Governor Rick Perry and Wisconsin Governor Scott Walker, were similarly critical of Trump.

The Republican National Committee said: “There is no place in our party or our country for comments that disparage those who have served honourably.”

Veterans groups also piled in. Paul Rieckhoff, founder of Iraq and Afghanistan Veterans of America, said Trump’s “asinine comments” were “an insult to everyone who has ever worn the uniform – and to all Americans”.

Despite the criticism, Trump is under no pressure to withdraw from the race because the billionaire businessman is paying for his own campaign and does not need to rely on wealthy donors.