Trump, an unknown quantity in first Republican debate

As nine candidates prepare to take their place Thursday alongside Donald Trump for the first Republican presidential debate, a question looms: how does one deal with a rival whose bombast is part of his DNA?

The real-estate tycoon turned White House contender is also the United States' most prolific political insult generator, imposing a sense of must-see-TV on the opening duel of the 2016 cycle.

He blasted fellow Republican White House hopeful Jeb Bush as "weak," Senator Lindsey Graham as an "idiot," and Texas ex-governor Rick Perry as a buffoon who wears glasses "so people will think he's smart."

And that was in just one campaign speech.

Despite or perhaps because of his outrageous promises and harsh invective since launching his campaign in June, the brash billionaire has snatched a wide poll lead.

Trump has trumped many who have spent years meticulously preparing for this moment -- writing memoirs or policy books, espousing causes, polishing their image and courting wealthy donors.

Should they now hit back at Trump to get noticed, but at the risk of legitimizing a man the Republican National Committee has begged to tone it down?

Ammunition abounds: Trump in the past has supported abortion rights and universal health care -- heresy for the Republican base -- and has backed Democrats like Hillary Clinton.

But rivals would note that taking potshots at Trump could trigger a response in kind.

Bush quipped he was reassured that retired neurosurgeon Ben Carson will participate in the debate because "before that thing's over we might just need a doctor."

Graham, who branded Trump a "jackass," insisted Trump might make more sense to debate viewers if they were drunk. New Jersey Governor Chris Christie vowed not to let anyone interrupt him on stage.

A veteran of primary debates, Senator and former Republican presidential nominee John McCain, recommended sobriety.

But "I clearly would challenge some of his assertions," McCain told AFP. "Prepare to discuss the issues."

Carmine Gallo, a communications expert and author of "Talk Like TED," a reference to the renowned speakers' series, offered advice for those challenging The Donald: "Stay in your lane."

"Above all, you can't out-Trump Trump," he said. "Americans like their heroes to be authentic and you'll look like a phony if you try to match someone else's personality."

Winning candidates, he said, "are often those who paint an irresistible portrait of a bright future. Americans like optimists."

- Trump against type? -

University of Missouri political communication professor Mitchell McKinney, who has studied voter reactions for decades, said debut debates are essential for forming opinions about candidates.

That rings more true this year with 17 Republicans, including lesser-known personalities, vying for the nomination.

The top 10 in recent national polls will take the stage Thursday, with the remaining seven relegated to a forum four hours before the main event.

"This is their first sort of job interview with the party faithful," McKinney said.

Viewers will be looking for the suitable temperament of a future president more than a propensity to fling zingers at Trump.

"Can they attack an opponent, a fellow Republican, and do it in a way that they don't seem hateful or mean-spirited? Or do they seem like they're nothing but a bully," posed McKinney.

"Most folks don't find that quality particularly endearing in a president."

Candidates who cross into bullying behavior risk backlash from voters.

In their 2000 debate, Republican Rick Lazio interrupted fellow Senate candidate Hillary Clinton by walking to her podium, campaign finance pledge in hand, and urging her to sign it.

A heated 2011 primary debate turned physical when a frustrated Mitt Romney, the eventual Republican nominee, placed his hand on Perry's shoulder.

Trump shines when he faces the cameras alone. It may be different when he shares the stage, allotted the same time as nine rivals.

Trump dialed down the loose-cannon threat Sunday, telling ABC News: "I'm not looking to attack them."

He also downplayed expectations of his own performance, stressing: "I'm not a debater."

The biggest bombshell could come if Trump goes against type, emerging as thoughtful, composed and not lobbing verbal grenades.

"I actually predict that Mr Trump will be focused on policy and respectful," grinned Republican Senator Rob Portman, who played the role of President Barack Obama in Romney's 2012 debate prep.

Fox News released the debate line-up Tuesday.

Joining Trump will be: Bush; Wisconsin Governor Scott Walker; ex-Arkansas governor Mike Huckabee; Carson; Senators Ted Cruz, Marco Rubio and Rand Paul; Christie; and Ohio Governor John Kasich.