Meanwhile, neurosurgeon Ben Carson is gaining momentum -- now the only candidate even close to Trump -- who has topped 30 percent in recent national polls and continues to lead in early states.
Here is a look at the key moments at the second presidential debate:
1. Did Carly Win?
Carly Fiorina stole the show and possibly won the debate.
“This is about the character of our nation, and if we will not stand up and force President Obama to veto this bill, shame on us,” Fiorina said, her voice raising and referring to the bill that is looking to defund Planned Parenthood.
Planned Parenthood has said the videos are “heavily edited” and has denied wrongdoing.
Shortly after the exchange, Tapper asked Fiorina about Trump’s comments to Rolling Stone magazine where he said, “Look at that face. Would anyone vote for that? Can you imagine that, the face of our next president?”
Fiorina’s retort, to huge cheers from the crowd: “You know, it's interesting to me, Mr. Trump said that he heard Mr. Bush very clearly and what Mr. Bush said [about women's health issues]. I think women all over this country heard very clearly what Mr. Trump said.”
Trump responded, “I think she's got a beautiful face and I think she's a beautiful woman.” Fiorina didn’t change her facial expression, but with a slight eye-roll she clearly won the exchange.
2. Donald Trump Insult-Fest
Trump fired back after a Paul attack: “I never attacked him on his looks, and believe me, there’s plenty of subject matter.”
He didn’t stop there, throwing out a snide side remark to Jeb Bush, standing next to him. “More energy tonight. I like that.” Trump has repeatedly hit the former Florida governor on his energy level, attacking him for having “low energy” on the campaign trail.
3. Jeb Bush Admits He Smoked Weed
“So 40 years ago, I smoked marijuana,” he said. “I'm sure other people might have done it and may not want to say it in front of 25 million people. My mom’s not happy that I just did.”
He went on to call drugs in the United States “a serious problem.”
Fiorina later jumped onto the issue, telling the audience that her daughter died because of a drug addiction. “We are misleading young people when we tell them that marijuana is just like having beer. It is not,” she said.
“And the marijuana that kids are smoking today is not the same as the marijuana that Jeb Bush smoked 40 years ago,” she continued. “We do -- sorry, Barbara.”
4. Trump’s the Target
Trump seemed to go after all of his opponents, but he also had a target on his back.
Rand Paul questioned Trump’s “judgment” when he immediately attacked the Kentucky senator.
“Do we want someone with that character, that kind of careless language to be negotiating with Putin? Do we want someone like that to be negotiating with Iran?” Paul asked. “That happened in junior high. Are we not way above that? Would we not all be worried to have someone like that in charge of the nuclear arsenal?” Paul asked.
Not all of his rivals immediately took the opportunity to go after the real estate mogul. Fiorina called Trump a “wonderful entertainer,” but punted on whether the nuclear codes were safe in his hands. Bush at first also balked saying he thinks “voters will make that determination.”
Walker went back at Trump when he was attacked saying, “Mr. Trump, we don't need an apprentice in the White House, we have one right now.”
5. Carly Fiorina Rolls On Foreign Policy
Fiorina had one message in her answers on foreign policy: I’ve done my homework.
The former Hewlett-Packard CEO aimed to show that her outsider status doesn’t mean she’s inexperienced on foreign policy.
“Having met Vladimir Putin, I wouldn't talk to him at all. We talked way too much to him,” she said, going on to talk about Russia, Germany, Iran and Syria, as well as Putin and General Qasem Soleimani.
Fiorina, who previously chaired the CIA’s external advisory board, plowed through an effort from moderator Jake Tapper to stop her. “We could rebuild the sixth fleet. I will. We haven't. We could rebuild the missile defense program. We haven't. I will.”
6. The Trump-Bush Battle Continues
In the long-standing feud between Trump and Bush, things got personal when the moderator, CNN’s Jake Tapper, asked if Trump went too far in attacking Bush’s wife.
"He did. He did,” Bush said. "You're proud of your family as I am. To subject my wife into the middle of political conversation was completely inappropriate and I hope you apologize for that.”
Though Trump called Columba Bush a “lovely woman”, he refused. “I won't do that because I said nothing wrong,” Trump said.
It all started when Trump told CNN in July that "If my wife were from Mexico, I think I would have a soft spot for people from Mexico.” Bush’s wife, Columba is Mexican-born, and emigrated to the country legally.
“My wife is a Mexican-American. American by choice,” Bush said at the debate. “She loves this country as much as anybody in this room and she wants a secure border and wants to embrace the American values that make us special and unique.”
For her part, Columba Bush tweeted her response. "I came to America because Ilove this country. @realDonaldTrump is wrong"
7. Was Marco Rubio the Sleeping Giant?
Marco Rubio may not have had any viral moments, but every time he was in the spotlight he stole it making him the possible dark horse standout of the GOP debate.
He knocked Trump’s foreign policy knowledge, calling the topics Trump got wrong in a previous interview with Hugh Hewitt “important issues” for a presidential candidate. “The next president of the United States better be someone that understands these issues,” Rubio said.
Rubio was also asked about Trump’s comment that Bush should not be speaking Spanish on the campaign trail, emotionally invoking his family history.
“I agree English is the unifying language of our country,” Rubio said before speaking about his grandfather. “I’ll tell you a story about somebody that didn't know English well: my grandfather.” He continued: “My grandfather instilled in me the belief I was blessed to live in one society where even I, the son of a bartender and maid could aspire to have anything and be anything.”
8. The Time Bush Looked Like He Was Going to Slug Trump
Bush let loose when Trump went after Bush’s brother, former President George W. Bush.
The former Florida governor was asked by moderators about his last name -- and what his family legacy meant for his candidacy.
Trump told Bush “your brother and your brother's administration gave us Barack Obama, because it was such a disaster those last three months that Abraham Lincoln couldn't have been elected.”
Bush answered, to a roaring applause: “You know what? As it relates to my brother, there is one thing I know for sure, he kept us safe.”
9. Scott Walker: Now You See Him, Now You Don’t
Walker, looking to recover from a nosedive in the polls that followed his forgetful performance in the first debate, was notably more aggressive and passionate when he took the stage at this debate. But he continued to struggle to make his voice heard.
Walker’s voice was close to absent for long portions of the marathon session. And, at one point, the timer clocked more than 30 minutes without one word uttered by the Wisconsin governor.
At the beginning of the debate, Walker tore into Trump: “We don’t need another apprentice in the White House,” he said.
He also pushed his way into the conversation at times, engaging on questions that were not directed at him. And at another point, Walker did something else he didn’t do in the last debate — he talked over a moderator.