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 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.

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


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

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".

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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," Fiorina said, drawing applause.

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

Fiorina has been rising in the polls and was sharing the stage with the leading candidates for the first time.

She noted Trump's companies had filed for bankruptcy four times and asked, "Why should we trust you?"

Bush, a frequent target of 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.


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

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."

Bush, an establishment Republican, has been overshadowed by 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.

Trump said Paul did not even belong on the stage because of his low standing in the polls, but Paul, a senator from Kentucky, criticised 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," Trump said.

Paul took a veiled swipe at Bush, saying "someone onstage" admitted to smoking pot in high school, showing how rich kids can smoke pot but black kids get thrown in jail for it.

Reuters/Ipsos opinion polling shows Trump leading among Republicans with 32 percent.

Retired neurosurgeon Ben Carson is second at 15 percent. Bush, the younger brother of former President George W Bush, is in third place at 9 percent as his campaign struggles to take off.

Source: Reuters