Has Carly Fiorina found Donald Trump's kryptonite?

Media watch an exchange between US Republican presidential candidates Donald Trump and Carly Fiorina during the second US Republican Presidential candidates debate at the Ronald Reagan Presidential Library in Simi Valley, California, USA, 16 September 2015. Image copyright EPA

The BBC's Jon Sopel sees presidential hopeful Donald Trump roll with the punches in the second Republican debate in California - until a new competitor entered the ring.

In the Victorian fairground where the grotesque mixed with the gaudy, where the sound of the barrel organ mixed with the whooshes and bangs of the various rides, there would be the stand where you would be able to lace up a pair of boxing gloves and take on the gnarled and grizzled man mountain who would be waiting for you in the ring.

And the crowd would "ooh" and "aah" as the prizefighter set about you, raining blows down above and below the belt.

And so it was in the beautiful Simi Hills in California last night at the Ronald Reagan Presidential Library that the Republican candidates took it in turn to try to give Donald Trump a bloody nose, a cracked rib, a thick lip and black eye. Ideally all four at once.

First up, flexing his muscles, and jabbing hard, was Senator Rand Paul on whether Trump had the character to have his hand on the nuclear trigger.

Image copyright EPA
Image caption Rand Paul was the first to attack Trump - but not the first to draw blood

"I'm very concerned about him - having him in charge of the nuclear weapons, because I think his response, his - his visceral response to attack people on their appearance - short, tall, fat, ugly - my goodness, that happened in junior high. Are we not way above that?"

Good shot, Rand, but thwack came the response - "I never attacked him on his look, and believe me, there's plenty of subject matter right there," said Trump.

Cue laughter. Next up into the ring was Wisconsin governor Scott Walker. "Mr Trump. We don't need an apprentice in the White House." But with a squint of those blue eyes, Trump shot back "in Wisconsin, you're losing $2.2bn right now. I would do so much better than that."

Jake Tapper, the excellent CNN host, didn't quite say "roll up, roll up who else wants to take their chances against my boy?" But he didn't need to. They were lining up.

Hold on a minute. What's this? My, my, a lady is getting into the ring. Surely she wouldn't be able to draw blood where everyone else had failed. Well she did.

The former Hewlett Packard boss Carly Fiorina had been roundly insulted by Trump when he questioned her suitability because of her appearance.

Image copyright AP
Image caption A handshake after the debate

He told Rolling Stone - "Look at that face. Would anyone vote for that? Can you imagine that, the face of our next president?"

She was stoic and measured: "I think women all over this country heard very clearly what Mr Trump said." And Trump was now visibly back-pedalling. "I think she's got a beautiful face, and I think she's a beautiful woman."

Her face was acid. She did not flinch. It was a gotcha moment.

And "low energy" Jeb Bush - as Donald Trump dubbed him - achieved something similar when he gave a Trump a good kicking over the tycoon bringing Bush's Mexican-born wife into the political debate.

Trump looked defensive. He wouldn't give the apology that Bush demanded, but he was discomfited.

There was a lot of serious stuff in this sprawling three hour debate. But if you're in a fairground - well you want all the fun of the fair. And once again Donald Trump provided it. He is still the focal point. But he saw tonight that his opponents are prepared to fight back.

And so the circus moves on. So roll up next time to see whether the polls change, whether all those candidates can stay the course. Thrills and spills await us.