Republican front-runner Donald Trump came under heavy attack from his rivals in a contentious U.S. presidential debate on Wednesday, and former Hewlett-Packard Chief Executive Carly Fiorina led the charge.

The most tweeted moment of the debate was her rebuttal to Trump’s statement in a recent Rolling Stone article that her face made her unelectable, with Facebook also registering the exchange as its top social moment.

"I think women all over this country heard very clearly what Mr. Trump said," Fiorina said, drawing applause.

Trump walked back his earlier statement by replying, "I think she's got a beautiful face, and I think she's a beautiful woman."

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Former Florida Gov. Jeb Bush also confronted Trump and grew stronger as the debate wore on, sharply defending his brother, former President George W. Bush, when Trump criticized the war in Iraq.

“[My brother] kept us safe,” Bush said, to which Trump responded, “Do you feel safer now? I don’t feel so safe.”

Bush, a frequent target of Trump's jibes for his "low-energy" campaign, criticized the billionaire for his past friendliness with Democrats and for trying to get involved in casino gambling in Florida.

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Reveling in the spotlight, Trump fired off insults, declaring that former New York Gov. George Pataki "couldn't get elected dogcatcher," and saying of Kentucky Sen. Rand Paul, "I never attacked him on his looks, and believe me, there's plenty of material there.”

When he was asked what his Secret Service code name would be if elected, Trump responded, “Humble.”

A breakdown of speaking times in the debate showed Trump led the way with more than 19 minutes, Bush was second with nearly 16 minutes and Fiorina was third with  approximately 14 minutes.