SIMI VALLEY, California (REUTERS) - Republican front runner Donald Trump traded jabs with former Florida governor Jeb Bush, former business executive Carly Fiorina and other presidential candidates in a contentious debate on Wednesday (Sept 16) marked by bickering and personal attacks.

In the second debate of the 2016 Republican White House race, the candidates also disagreed on whether the government should be shut down in order to cut funds for the women's health group Planned Parenthood and condemned President Barack Obama's nuclear deal with Iran.

But the focus remained on Mr Trump, the real estate mogul and former reality-TV star who has shot to a big lead in opinion polls in the 16-strong Republican presidential race.

Mr Trump touted his business experience and fired off insults, making a crack about US Senator Rand Paul's looks and declaring that former New York governor George Pataki "couldn't get elected dogcatcher".

Mr Trump drew a sharp rebuke from former Hewlett-Packard chief executive Fiorina for his recent comment in an interview that voters might not back her because of her face.

"I think women all over this country heard very clearly what Mr Trump said," Ms Fiorina said, drawing applause.

The tycoon leaned towards her and replied: "I think she's got a beautiful face, and I think she's a beautiful woman."

Ms Fiorina has been rising in the polls and was sharing the stage with the leading candidates for the first time. She noted Mr Trump's companies had filed for bankruptcy four times and asked: "Why should we trust you?"

Mr Bush, a frequent target of Mr Trump's jibes for his"low-energy" campaign, criticised the billionaire for his past friendliness with Democrats and for trying to get involved in casino gambling in Florida.

"He asked Florida to have casino gambling and we said no," Mr Bush said.

Mr Trump shrugged his shoulders dismissively and denied trying to get into the casino industry in Florida. "Wrong... Jeb, come on." He added: "More energy tonight? I like that."

Mr Bush, an establishment Republican, has been overshadowed by Mr Trump's bomb-throwing rhetoric in the contest to become the party's candidate for the November 2016 presidential election but had promised to be more aggressive in the debate.

Mr Trump said Mr Paul did not even belong on the stage because of his low standing in the polls, but Mr Paul, a senator from Kentucky, criticised Mr Trump for his "sophomoric" criticisms of people's appearance.

"I never attacked him on his looks, and believe me there's plenty of material there," Mr Trump said.

'CHILDISH BACK AND FORTH'

At one point, New Jersey Governor Chris Christie derided the "childish back and forth" and called on the candidates to be more substantive.

Reuters/Ipsos opinion polling shows Mr Trump leading among Republicans with 32 per cent. Retired neurosurgeon Ben Carson is second at 15 per cent. Mr Bush, the younger brother of former president George W. Bush, is in third place at 9 per cent as his campaign struggles to take off.

Eleven candidates were on stage for the debate at the Ronald Reagan Presidential Library in Simi Valley.

Also debating in the main event were Mr Scott Walker, Mr Ted Cruz, Mr Marco Rubio, Mr Mike Huckabee, Mr John Kasich and Mr Chris Christie.

Mr Cruz, a US Senator from Texas, said the nuclear deal with Iran should be ripped up, but Mr Paul said it should be enforced because Congress had not been able to stop it.

On Planned Parenthood, Mr Kasich said it would not be worthwhile to risk a government shutdown in order to strip federal funding for the women's health group. "I would not be for shutting the government down, because I don't think it's going to work out," Mr Kasich said.

Mr Cruz, who led the effort to shut down the government over funding for Obamacare, said, "We need to stop surrendering and start standing for our principles."

Before the prime-time encounter, Mr Rick Santorum, Mr Bobby Jindal, Mr George Pataki and Mr Lindsey Graham participated in the "undercard" debate for those candidates who did not poll strongly enough to qualify for the main event.

Mr Jindal, the Louisiana governor, and Mr Pataki, the former New York governor, led the early charge against Mr Trump.

"Let's stop treating Donald Trump like a Republican," said Mr Jindal. "He's not a conservative. He's not a liberal. He's not a Democrat. He's not a Republican. He's not an independent. He believes in Donald Trump."

Mr Pataki, who has vowed not to support Mr Trump if he is the nominee, insisted the tycoon would not get that far.

"Donald Trump is not going to be the Republican nominee, period. I guarantee you that," Mr Pataki said.