At the CBS Republican primary debate in Greenville, S.C., candidates weighed in on the passing of Supreme Court Justice Antonin Scalia. (CBS)

At the CBS Republican primary debate in Greenville, S.C., candidates weighed in on the passing of Supreme Court Justice Antonin Scalia. (CBS)

The six remaining Republican presidential candidates sparred with ferocity over U.S. foreign policy in a debate here Saturday night, with front-runner Donald Trump savaging former president George W. Bush’s intervention in Iraq, which helped spawn more than a decade of instability in the Middle East.

In the most impassioned argument yet this year over the Bush legacy, Trump effectively set the past generation of Republican foreign policy ablaze by assailing the doctrine of forceful intervention, echoing views long held by many Democrats and drawing a fiery rebuke from Bush’s brother, Jeb.

Trump, using visceral and vitriolic language, blamed the 43rd president for not keeping the country safe from the terrorist attacks of Sept. 11, 2001, and alleged that he lied to the nation about the existence of weapons of mass destruction in Iraq.

“Obviously the war in Iraq was a big, fat mistake,” Trump thundered. “George Bush made a mistake. We can make mistakes, but that one was a beauty. We should have never been in Iraq. We have destabilized the Middle East.”

Trump’s assault came as the former president is scheduled to return to the political arena Monday in North Charleston, S.C., for a campaign rally with his brother. Trump’s offensive sparked an electric series of exchanges with Jeb Bush, who condemned the billionaire mogul as an empty showman who lacks a serious worldview.

Republican presidential contender Ted Cruz and CBS GOP debate host John Dickerson sparred over when Supreme Court Justice Anthony Kennedy was nominated and confirmed at the CBS News GOP debate on Feb. 13, 2016. (CBS)

Bush accused Trump of mocking and tormenting his family as “blood sport.”

“I am sick and tired of him going after my family,” said Bush, a former Florida governor. “While Donald Trump was building a reality TV show, my brother was building a security apparatus to keep us safe.”

Bush’s rejoinder drew an endorsement from Sen. Marco Rubio (Fla.), who has an otherwise icy relationship with his onetime Florida mentor. “I thank God all the time that it was George W. Bush in the White House on 9/11 and not Al Gore,” Rubio said. “He kept us safe.”

Trump jumped in: “The World Trade Center came down during the reign of George Bush. He kept us safe? That is not safe.”

The crowd, dominated by hundreds of South Carolina Republican activists and donors, loudly booed Trump at various points. The front-runner pooh-poohed the audience, dismissing them as “Jeb’s special interests and lobbyists.”

Trump tried to channel both the frustrations of many Americans who are weary of more than a decade of war and the fears that people have about the threats posed by Islamic State terrorists.

“You have to knock out ISIS. They’re chopping off heads. These are animals. You have to knock ’em out. You have to knock them off strong,” he said. But, Trump added, “We’ve been in the Middle East for 15 years and we haven’t won anything.”

1 of 25 Full Screen Autoplay Close Skip Ad × Republican presidential candidates clash during CBS News debate in South Carolina View Photos The six remaining GOP candidates differed sharply on U.S. foreign policy, just hours after learning of the death of Supreme Court Justice Antonin Scalia. Caption The six remaining GOP candidates differed sharply on U.S. foreign policy just hours after learning of the death of Supreme Court Justice Antonin Scalia. Feb. 13, 2016 Ohio Gov. John Kasich, left, Jeb Bush, Sen. Ted Cruz (Tex.), Donald Trump, Sen. Marco Rubio (Fla.) and Ben Carson take the stage beneath an image of Supreme Court Justice Antonin Scalia, who died earlier in the day, for the Republican debate in Greenville, S.C. Erik S. Lesser/European Pressphoto Agency Buy Photo Wait 1 second to continue.

It was a remarkable moment at a critical point in the topsy-turvy Republican nominating battle. Summing up the discussion, Ohio Gov. John Kasich said: “I got to tell ya, this is just crazy. This is just nuts. Geez, oh, man.”

Later, bemoaning the frequently personal attacks, Kasich preached Republican unity. “I think we’re fixing to lose the election to Hillary Clinton if we don’t stop this,” he said, referring to the Democratic presidential hopeful.

The CBS debate, staged at the Peace Center in downtown Greenville and moderated by “Face the Nation” anchor John Dickerson, was the last before next Saturday’s South Carolina primary. The stakes are high: The field is likely to continue to shrink after South Carolina, with special pressure on three establishment-friendly candidates — Bush, Kasich and Rubio — duking it out for supremacy.

Each candidate tried to prove Saturday night that he belongs in the Oval Office.

There was intense pressure on Rubio, who has struggled to recover from his floundering performance in last Saturday’s debate in New Hampshire, to return to his prior form as an agile and skilled debater. From his first answer, Rubio sought to project strength and command of the issues, speaking in detail about foreign policy challenges in Asia, Europe and the Middle East.

Ben Carson, a retired neurosurgeon who has struggled to demonstrate knowledge about foreign affairs, stressed that he had exhibited sound judgment in the operating room.

“As far as that 2 a.m. phone call is concerned, judgment is what is required, and the kinds of things that you come up with are sometimes very, very difficult, and very unique,” Carson said.

Four hours before the debate began, a jolt arrived with the news that Supreme Court Justice Antonin Scalia had died. The debate opened with the candidates bowing their heads in a moment of silence to remember the jurist.

Scalia’s death instantly loomed over the presidential race, injecting a new level of seriousness into the conversation and reminding voters that the next president’s appointments could reshape the nation’s highest court.

The candidates realized that, for the first time in this campaign, a Supreme Court vacancy was not just an abstract possibility but a reality.

The candidates took turns praising the conservative justice and vowing that President Obama should not be allowed to push through a nomination to fill the vacancy before the end of his term.

“I think it’s up to [Senate Majority Leader] Mitch McConnell [R-Ky.] and everybody else to be able to stop it,” Trump said. “It’s called delay, delay, delay.”

Kasich lamented the gridlock that could be created by a nomination fight.

“I just wish we hadn’t run so fast into politics,” he said. “The country is so divided right now, and now we’re going to see another partisan fight taking place.”

Dismissing Obama as a “lame-duck president,” Rubio said, “Someone on this stage will get to choose the balance of the Supreme Court, and it will begin by filling this vacancy that’s there now.”

Each candidate made clear that he wants justices who consider the Constitution a sacred document to be upheld and protected.

Sen. Ted Cruz of Texas, a former Supreme Court clerk, called Scalia “a legal giant.”

“He was a brilliant man,” Cruz said. “He was faithful to the Constitution. He changed the arc of American legal history.”

Cruz suggested that he would appoint a justice in Scalia’s mold.

“We are one justice away from a Supreme Court that will strike down every restriction on abortion adopted by the states,” Cruz said. “We are one justice away from a Supreme Court that will reverse the Heller decision [on the Second Amendment right to bear arms]. . . . We are one justice away from a Supreme Court that would undermine the religious liberty of millions of Americans.”

Cruz attacked Trump over the businessman’s past liberal positions on abortion and other social issues. He also criticized Trump for asserting that, outside abortions, Planned Parenthood did important work for women’s health.

“You are the single biggest liar,” Trump thundered at Cruz at one point. “This guy will say anything. Nasty guy. Now I know why he doesn’t have one endorsement from any of his colleagues.”

Cruz responded: “Donald has this weird pattern. When you point to his own record, he screams, ‘Liar, liar, liar.’ ”

During the debate, Cruz and Rubio yet again tangled aggressively over immigration. Rubio repeatedly accused Cruz of once supporting a pathway to citizenship for those now illegally in the country, a position that Cruz denies ever holding.

Cruz then fired back at Rubio for his changing positions on immigration issues, including for his previous support of in-state college tuition for illegal immigrants, and taking different positions in Spanish-language interviews from those expressed in English-language interviews.

“I don’t know how he knows what I said on Univision because he doesn’t speak Spanish,” Rubio snapped back derisively.

Trump was given the chance to get into the immigration debate and claimed credit for placing the issue at the forefront of the presidential campaign. Then he turned to Bush and attacked him, labeling him the weakest of the candidates on immigration.

Bush fired back that Trump was “weak” for having disparaged women, Hispanics, the disabled as well as Sen. John McCain (R-Ariz.), a Vietnam War hero.