The former Hewlett-Packard chief executive brazenly confronted Donald Trump on the debate stage Wednesday night, putting the front-runner in the rare position of being on defense.

She was stern when asked about Trump's recent assessment of her appearance, when he told Rolling Stone: "Look at that face! Would anyone vote for that?"

Fiorina shot back during the debate: "I think women all over this country heard very clearly what Mr. Trump said."

The exchange left Trump in an unusual position: struggling to find the right words to respond.

"I think she's got a beautiful face and I think she's a beautiful woman," he said, declining to take the opportunity to hit back.

Trump also faced withering attacks from former Florida Gov. Jeb Bush and Sen. Rand Paul. The jabs marked a shift in tone for Republican candidates since the last debate in August, when many of Trump's competitors were hesitant to directly take him on. By Wednesday, those reservations were gone, a sign of how seriously his rivals are taking Trump's candidacy.

The debate also served as a forum for several candidates to speak passionately about Planned Parenthood and vow to defund the group in the wake of controversial and highly edited videos of organization officials discussing the sale of aborted fetal parts.

"These Planned Parenthood videos are horrifying," said Sen. Ted Cruz, as he accused the organization of trying to "sell the body parts of unborn children for profit."

New Jersey Gov. Chris Christie responded: "Let's ask Hillary Clinton. She believes in the systematic murder of children in the womb."

Fiorina used even more graphic language.

"Watch these tapes. Watch a fully formed fetus on the table. Its heart beating, its legs kicking while someone says we have to keep it alive to harvest its brain," she said.

Though Trump didn't pack as much punch as usual, he still found moments to go on the offense. He pounced on Fiorina's controversial tenure as head of Hewlett-Packard.

Her leadership at the firm "led to the destruction of the company," Trump said. "She can't run any of my companies -- that I can tell you."

Bush was uncharacteristically combative with Trump, seeming determined to prove that he could hold his own against the real estate mogul.

Asked to respond to Trump's controversial remarks about Bush's wife — "If my wife were from Mexico, I think I would have a soft spot for people from Mexico" — Bush gestured to his wife in the audience and asked for an apology.

Trump declined, adding: "No, I won't do that because I said nothing wrong but I hear she's a lovely woman."

Earlier in the evening, Bush had accused Trump of buying influence.

"You got Hillary Clinton to go to your wedding," Bush said.

"Excuse me, Jeb, I got along with Clinton, I got along with everybody," Trump said. "Excuse me."

Jeb cut in with a curt: "No."

"More energy tonight -- I like that," joked Trump, who has been criticizing Bush relentlessly for lacking in charisma.

As he fended off one attack after another, Trump — well-known for his insatiable appetite for mocking his critics — seemed to lack the fire that fueled his performance at last month's debate.

The debate came at a crucial point in the GOP primary, with tensions running high and establishment candidates struggling against their outsider counterparts.

"There's a sophomoric quality that is entertaining about Mr. Trump," said Kentucky Sen. Rand Paul, scolding Trump for going after people's appearance.

Trump responded: "I never attacked him on his looks and believe me there's plenty of subject matter right there."

Wisconsin Gov. Scott Walker also took an early shot at Trump. "We don't need an apprentice in the White House -- we have one right now."

Meanwhile, at an earlier debate for the GOP field's lower-ranking candidates, Louisiana Gov. Bobby Jindal, former New York Gov. George Pataki, South Carolina Sen. Lindsey Graham and former Pennsylvania Sen. Rick Santorum debated thorny political issues such as immigration.Trump has made the issue a central part of his campaign rhetoric -- and national security.

When the conversation turned to the controversial issue of "birthright citizenship," Graham said there were certain "rich Asians, rich people from the Mideast" that were "bastardizing citizenship."

Jindal, meanwhile, defended his policy views on immigration, repeatedly asserting that he did not support amnesty.

In another heated exchange, Pataki and Santorum butted heads about a Kentucky county clerk's refusal to issue marriage licenses to same-sex couples.

The woman, Kim Davis, has reignited a national debate about a Supreme Court ruling earlier this year that legalized same-sex marriage across the country.

Santorum called the Supreme Court ruling "unconstitutional," and said there is no more important right than the ability for a citizen to freely exercise his or her conscience.

But Pataki said he would have fired Davis for violating the law.

"I didn't agree with the Supreme Court's decision but it is the law of the land," Pataki said.

While the four candidates traded barbs over numerous issues, there were also calls for the GOP to focus on the ultimate prize of taking back the White House.

"If it is Hillary Clinton, Joe Biden or Bernie Sanders (that becomes president), they're going to pick people we're going to disagree with all the time," Graham said in reference to Supreme Court nominations. "Please understand that we have to win this election."

(CNN)