Ted Cruz, left, and Donald Trump at the debate. REUTERS/Jonathan Ernst Donald Trump went after Sen. Ted Cruz (R-Texas) late in Saturday night's CBS Republican debate while arguing about Trump's more liberal past.

Cruz said Trump was "very liberal" on social issues, causing Trump to fire back that Cruz was the "single biggest liar."

It was perhaps the highlight of a fiery presidential debate in which the GOP candidates repeatedly attacked each other on stage.

Trump unloaded on former Florida Gov. Jeb Bush (R) multiple times. Cruz and Sen. Marco Rubio (R-Florida) traded blows. Bush went after Ohio Gov. John Kasich (R).

"Seriously, this is insane. The GOP is destroying itself tonight, and they have no one to blame but themselves," veteran Republican pollster Frank Luntz wrote on Twitter.

Some of the last fireworks of the night came when Cruz attacked Trump's supposedly "very, very liberal" policies.

"Flexibility is a good thing, but you shouldn't be flexible on core principles," Cruz said. "I like Donald. He is an amazing entertainer. But his policies for most of his life ... have been very, very liberal."

Cruz continued:

For most of his life, he has described himself as very pro-choice and as a supporter of partial-birth abortion. Right now today as a candidate, he supports federal taxpayer funding for Planned Parenthood. I disagree with him on that.

"You are the single biggest liar," Trump shot back. "You probably are worse than Jeb Bush. ... This guy lied about Ben Carson when he took votes away from Ben Carson in Iowa and he just continues."

Trump also accused the Cruz campaign of running robo-calls saying that Trump was not going to run in the South Carolina primary:

Today, we had robo-calls saying, "Donald Trump is not going to run in South Carolina," where I'm leading by a lot. ... "Vote for Ted Cruz." This is the same thing he did to Ben Carson. This guy will say anything. Nasty guy. Now I know why he doesn't have one endorsement from any of his colleagues.

Moderator John Dickerson cut in and told Cruz to "just pick from the buffet there" in order to issue his response to the attacks.

"I will say it is fairly remarkable to see Donald defending Ben after he called him pathological and compared him to a child molester, both of which were offensive and wrong," Cruz replied.

Trump and Rubio. REUTERS/Jonathan Ernst

Cruz continued by saying that Trump "didn't disagree with the substance that he supported taxpayer funding for Planned Parenthood."

"Donald has this weird pattern," Cruz added. "When you point to his own record, he screams, 'Liar, liar, liar.'"

Trump started shouting over him, saying, "Where did I support it? Where did I support it?"

After some more back and forth over Planned Parenthood, Cruz said that if Trump were president, the Second Amendment would "go away" and that Trump would appoint liberals to the Supreme Court. The attack came hours after Justice Antonin Scalia's sudden death earlier in the day.

As Trump and Cruz kept going at it, Carson cut in and said, "My name was mentioned twice." Carson indicated that he wanted to respond. Bush also cut in and noted that Trump had called him a "liar," but Dickerson wanted to move on.

"We're in danger of driving this into the dirt," Dickerson said.

He was too late. He invited Rubio to jump in, but Bush kept talking.

"He also denigrated one of my heroes, Ronald Reagan," Bush said.

Dickerson went back to Cruz, who fired back at Trump for highlighting Cruz's past support for Chief Justice John Roberts. Trump pointed out that Roberts had a decisive role in the two rulings upholding the constitutionality of crucial parts of Obamacare.

"I did not nominate John Roberts," Cruz said. "I would not have nominated John Roberts."

"You pushed him. You pushed him," Trump said. "You worked with him and you pushed him."

Trump and Cruz kept shouting over each other.

"Donald, Donald, Donald, adults learn not to interrupt each other," Cruz said.

"Why do you lie?" Trump asked, adding sarcastically: "Yeah, yeah, I know. You're an adult."

Earlier in the debate, Trump and Cruz went back and forth in a wild exchange that had Trump questioning the judgment of former President George W. Bush, Jeb Bush's brother, in the aftermath of the September 11, 2001, terror attacks.

"Obviously the war in Iraq was a big, fat mistake," Trump said. He then attacked Jeb Bush for waffling earlier in the campaign on whether the war was a mistake.

"It took him five days before his people told him what to say, and ultimately he said it was a mistake," Trump said.

Jeb Bush said that he was "sick and tired" of President Barack Obama blaming his brother for problems within his own administration.

Bush continued:

I could care less about the insults that Donald Trump gives to me. It's blood sport for him, he enjoys it, and I'm glad he's happy about it. But I am sick and tired, I am sick and tired of him going after my family. My dad [former President George H. W. Bush] is the greatest man alive in my mind. While Donald Trump was building a reality-TV show, my brother was building a security apparatus to keep us safe. And I'm proud of what he did.

"The World Trade Center came down during your brother's reign," Trump retorted. "Remember that."

The crowd booed loudly.