GOP debate: Donald Trump accuses presidential rival Ted Cruz of not being 'natural born' American

The billionaire New Yorker questioned the Texas senator's eligibility to stand for election as president

The Republican presidential candidates traded attacks during the GOP debate in South Carolina (Reuters)

Republican presidential rivals Donald Trump and Ted Cruz ended their truce in last night’s GOP debate, with the billionaire questioning whether the senator is even eligible to stand for election to become president.

The controversial New Yorker and Texas senator traded attacks at the debate in North Charleston – just two weeks before voters in Iowa pick their Republican and Democratic presidential choices in the first of the campaign’s caucuses.

Both men, who have been chasing the backing of the conservative Tea Party movement, had been on friendly terms prior to the debate.

But no niceties were exchanged before the audience in South Carolina, with Trump even questioning his rival’s constitutional claim to serve as president because he was born in Canada.

According to the US constitution only “natural born” citizens can become president, with this widely interpreted as requiring a candidate to have been born in the US or to a US citizen parent.

The 69-year-old frontrunner said: “Who the hell knows if you can even serve in office?

“There’s a big question mark over your head.”

Born in Calgary, Alberta, to a US citizen mother and a Cuban father, Cruz accused Trump of mentioning his birthplace simply because Cruz was leading some polls in Iowa.

The 45-year-old said Trump – who has also questioned whether Hawaiian-born President Barack Obama is really a US citizen – had asked his lawyers to look into the issue of Cruz’s birth in September and concluded there were no issues.

“Since September, the Constitution hasn't changed, but the poll numbers have,” Cruz retaliated.

“And I recognise that Donald is dismayed that his poll numbers are dropping in Iowa, but the facts and the law here are really clear.

“I’m not going to be taking legal advice from Donald Trump.”

The senator then questioned Trump on his apparent endorsement of New York’s liberal values.

He said: “Not a lot of conservatives come out of Manhattan – I’m just saying.

“Everyone understands that the values of New York City are socially liberal, are pro-abortion, are pro-gay marriage.”

Trump cited the 9/11 attacks in response and said that Cruz’s claim had insulted him.

“The people in New York fought and fought and fought, and we saw more death, and even the smell of death... And it was with us for months, the smell, the air,” he said.

“And we rebuilt Downtown Manhattan, and everybody in the world watched and everybody in the world loved New York and loved New Yorkers.

“And I have to tell you, that was a very insulting statement that Ted made.”

Additional reporting by Reuters