Trump targeted, Fiorina shines in US Republican debate
Carly Fiorina, the former chief executive of Hewlett-Packard, delivered a withering response to insulting comments about her looks that Donald Trump made in a recent magazine interview (AFP Photo/Frederic J Brown)

Republican presidential frontrunner Donald Trump stepped into a hornet's nest in Wednesday's debate as rivals turned their sights on the billionaire, while Carly Fiorina showed she had earned her place on the main stage.

Ten challengers flanked Trump on stage at the Ronald Reagan Presidential Library in California, seeking an opening against the man who has defied all political odds to lead the race for the party's nomination ahead of the November 2016 election.

"Mr. Trump, we don't need an apprentice in the White House, we have one right now," Wisconsin Governor Scott Walker told Trump -- a snarky reference to the title of the real estate mogul's reality television show.

Trump swiftly returned fire, living up to his billing as the campaign bulldog by attacking his rivals, further imposing himself on the race to determine who will battle the Democratic nominee, likely Hillary Clinton, for the presidency.

"In Wisconsin I went to number one, and you went down the tubes," Trump retorted, highlighting Walker's slumping poll numbers.

The remarks near the beginning of the marathon three-hour debate kicked off several minutes of Trump-related thrusts and parries that have so far defined much of the 2016 presidential race.

The event's 11 candidates clashed extensively on issues including immigration, how to handle a looming government shutdown, and dealing with Russia.

Many, like early presumptive frontrunner Jeb Bush, were under intense pressure to deliver a breakout performance -- or risk a campaign meltdown that could see them shunted aside as the first state nomination votes in February draw nearer.

- Command performance -

Fiorina, the sole woman in the Republican race, delivered by many accounts a command performance.

The former chief executive of technology giant Hewlett-Packard offered passionate calls for defunding women's health care provider Planned Parenthood, a publicly-funded organization that offers abortions.

She also delivered a withering response to Trump's insulting comments about her looks in a recent magazine interview, offering him a cold stare and earning loud applause.

Fiorina dominated the middle portion of the debate, showing command of military figures, slamming Clinton for what she called a lack of accomplishments, delivering an emotional call for increased drug treatment as she recalled losing a child to addiction and hitting Trump over his business practices.

"You ran up mountains of debt, as well as losses, using other people's money," Fiorina said.

"Why should we trust you to manage the finances of this nation?"

Bush, perhaps the campaign's ultimate establishment Republican, has seen his political fortunes tumble in the months since Trump entered the race, and he sought to claw back some of his lost ground by projecting himself as an even-keeled conservative who can lead from day one.

The two tangled as Trump again criticized the former Florida governor for speaking Spanish on the campaign trail. Here, Clinton jumped in via Twitter, writing in Spanish to defend the use of the language.

Bush also provided a witty retort to Trump's accusation that he is a "low-energy" candidate.

Asked what he would want his Secret Service handle to be if he were elected, Bush said: "Ever-ready. It's very high-energy, Donald."

- Awkward defence -

Bush was also forced into an awkward defense of his brother and former president George W. Bush, who launched divisive wars in Afghanistan and Iraq after the September 11, 2001 terror attacks.

"There is one thing I know for sure -- he kept us safe," Bush said.

"I don't know, do you feel safe right now?" Trump replied. "I don't feel so safe."

As more conventional candidates have struggled to secure precious air time with the media, another outsider, retired neurosurgeon Ben Carson, has quietly gained ground and now sits second in the polls.

While it is unclear whether Carson poses an immediate threat to Trump's dominance, the rise of the doctor -- who like Trump has never held public office -- is more evidence of an anti-establishment wave washing over the nomination race.

"When I entered this race, all the political pundits said it was impossible," Carson said. "We now have over 500,000 donations and the money is coming in."