WASHINGTON (AFP) - Carly Fiorina emerged as a true presidential contender in Wednesday's Republican primary debate, displaying both passion and a command of foreign policy - and landing stinging blows against frontrunner Donald Trump.

Steely-eyed and confident, the former Hewlett-Packard chief executive stole the show at times and won over the audience at the Ronald Reagan Presidential Library in Simi Valley, California, where 11 White House hopefuls engaged in a three-hour debate marathon.

Less than five months before voters cast ballots in the first nominating contests, the Republican field remains remarkably broad, with political outsiders Trump and retired neurosurgeon Ben Carson dominating opinion polls and the establishment candidates scrambling to make their mark.

Fiorina was elevated to the main debating event after her breakout performance in last month's "undercard" event held for candidates who are trailing in the polls.

And she managed to achieve what many of the other nine Trump rivals on stage have tried but ultimately failed to do: take the braggadocious billionaire down a notch and drag him out of his comfort zone.

She did so in front of a super-sized audience. Some 22.9 million people tuned in, making the debate the most watched program in CNN history, according to the network, which hosted the event.

WITHERING CRITIQUE

Trump, the populist outsider candidate in a year when Republican voter frustration with Washington has skyrocketed, has turned the campaign on its head and is leading all comers.

But he appeared subdued and out of his element when the discussion turned substantive, fading to the background for long stretches as his rivals clashed over the Iran nuclear deal or how to defeat the Islamic State.

And he was humbled - temporarily as it may be - by Fiorina's withering critique of Trump's insulting comments about her face that were reported earlier this month.

She also went after Trump's business dealings, saying he "ran up mountains of debt, as well as losses, using other people's money." "Why should we trust you to manage the finances of this nation?" she asked him.

'GREAT NIGHT' FOR FIORINA

While it seemed Fiorina found the Kryptonite to disable Trump, she also won plaudits for her moving account of losing a daughter to drug addiction and her passionate pro-life plea to end federal funding to women's health care and abortion provider Planned Parenthood.

"I think Carly had a great night," Republican National Committee spokesman Sean Spicer told CNN Thursday, although he declined to declare a debate winner.

Others did not hesitate.

"I'd be shocked if she doesn't keep rising in the polls," National Review editor Rich Lowry wrote of Fiorina in the conservative magazine.

There was praise as well for Senator Marco Rubio, who showed deft command of foreign policy priorities as well as compassion on how to address the issue of illegal immigration.

Rubio, Wisconsin Governor Scott Walker, New Jersey Governor Chris Christie and early presumptive frontrunner Jeb Bush were under intense pressure to deliver a breakout performance - or risk a campaign meltdown that could see them swept aside.

GANGING UP ON TRUMP

Several tugs of war between Bush and Trump highlighted the prickliness that marks their campaign rivalry.

Bush was forced into an awkward defence of his brother and former president George W. Bush, who launched divisive wars in Afghanistan and Iraq after the Sept 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."

While Trump weighed in only rarely on substantive debate over international issues - saying he would "get along with a lot of the world leaders that this country is not getting along with" - Bush along with Senator Rand Paul, Christie and others ganged up on Trump's policy inexperience.

"The lack of judgment and the lack of understanding about how the world works is really dangerous," Bush stressed.

Carson, the non-politician whose quiet poise is a stark contrast to some others on stage, had few standout moments, but he asserted himself when the discussion turned to campaign finance and charges that some candidates are beholden to big donors.

"I in no way am willing to get into bed with special interest groups or lick the boots of billionaires," Carson said.

On Thursday he offered morning-after praise for his rising rival Fiorina.

"We fought for Carly to be included in the debate. Political outsiders are winning the conversation," he posted on Twitter.