As the Republican field narrows, there are three big themes to watch for in Thursday night's GOP debate: Readiness, electability, and conservative values. WSJ's Jerry Seib explains. Photo: AP

DONALD Trump fiercely attacked his closest rival in a debate just weeks before Americans cast their first votes to select a Republican candidate to run for president.

Buoyed by a new poll that extended his lead over Ted Cruz, Trump delivered what many considered to be his most fiery debate performance to date, sharply attacking his rival’s eligibility to be President based on his Canadian birth.

Just hours before the debate, the new Wall St Journal/NBC poll had Trump opening a double-digit lead over second place runner Cruz — 33 per cent to 20 per cent.

In December, Trump was ahead by just five points — yesterday, it widened to 13.

It’s a crucial improvement with the first votes to be cast on February 1.

Trump was sidestepped by moderators for the first several questions of last night’s debate, but then opened with his hot-button issue of immigration.

Asked if he admits he stoked fear on refugees, he replied: “It’s not fear and terror, it’s reality.”

When asked specifically about Syrian refugees, he replied: “That could be the great trojan horse, it could be people who are going to do great, great destruction.”

“We owe 19 trillion dollars our county’s a mess and we can’t let all these people come into our countries and break our laws.”

Cheers filled the room.

Trump laboured the point of Cruz’s birth and eligibility to be President. He said he wouldn’t bring a law suit on the issue, but urged his opponent to get a declaratory judgment because someone would bring a law suit.

Cruz accused Trump of only attacking him because he had improved in the polls.

Trump admitted that was true. When asked by the moderators why he was focusing on the issue now, he retorted: “Because now he’s going a little bit better. Hey, look, he never had a chance. Now he’s doing better. He’s got probably a four or five per cent chance.”

Cruz fired back attacking Trump’s “New York values”.

When asked what New York values meant, Cruz replied that the people of South Carolina knew exactly what it meant.

“Everyone understands that the values in New York City are socially liberal or pro-abortion or pro- gay-marriage, focus around money and the media,” he said.

But it was an attack ultimately won by Trump, who fired back that New York rebuilt after the September 11 attacks, with the whole word “watched and loved” the city.

Jeb Bush also unleashed an attack on Trump, labelling his immigration policies “unhinged”.

He also said some of his plans — specifically tariffs on China — would be “devastating” for the economy.

Trump hit back with his regular attack on Bush, labelling him weak. “We don’t need a weak person being president of the US,” he said, adding that America already had a weak president.

Trump flew out of South Carolina for Iowa last night, the first US state to vote in the primary elections on February 1.

Trump: I’d give my businesses to my kids

Trump has revealed he would hand his business to his children if he was to become president.

Speaking at the Fox Business Network Republican Presidential debate, Trump said if he was to become president, he “couldn’t care less” about his company and personal wealth.

He said he would disentagle himself from his money so he could prioritise America’s interests.

“If I become president, I couldn’t care less about my company. It’s peanuts,” Trump said.

Trump said he would turn over his company’s operation to his children and top executives.

“Run the company, kids. Have a good time,” Trump said, motioning toward his son and daughter in the crowd.

Iran sanctions

Bush says the United States of American must rebuild its military, otherwise countries like Russia will not take it seriously.

Lamenting cuts to the navy military, Bush said some military planes are older than the pilots.

He also called on the U.S to re-impose sanctions on Iran as they have already violated its obligations following the recent nuclear deal by testing missiles.

It follows Cruz slamming Trump for his “New York values”, which seemed to offend the billionaire.

Cruz said “not a lot of conservatives come out of Manhattan. I’m just sayin’”

Trump described the comment as “a very insulting statement”, invoking September 11 as an example of the way New Yorkers came together after the attack.