Donald Trump gestures towards Senator Ted Cruz. Credit:Bloomberg As the audience expressed vocal disapproval, Mr Trump turned on its members. "You know who that is?" Mr Trump said of those booing. "That's Jeb's special interests, the lobbyists talking." Mr Bush, a persistent target of Mr Trump's insults, responded with uncharacteristic intensity. "This is from a guy who gets his foreign from the (Sunday morning news) shows," Mr Bush said. "This is from a guy who thinks Hillary Clinton is a great negotiator. ... This is a man who insults his way to the nomination. ... While Donald Trump was building a reality TV show, my brother was building a security apparatus to keep us safe." Mr Bush criticised Mr Trump's suggestion that the US partner with Russia to defeat Islamic State rebels.

Donald Trump looks toward Senator Marco Rubio during a break at the Republican presidential candidate. Credit:Bloomberg "The very basic fact is Vladimir Putin is not going to be an ally of the United States," Mr Bush said. "The world knows this." Mr Trump shot back that the "security apparatus" didn't do much good, pointing to the destruction of the World Trade Centre. Then he jabbed Jeb Bush for the huge amount he and his backers invested in the New Hampshire primary, only to place fourth. "$24 million in New Hampshire," Mr Trump said. "Gimme a break." Republican presidential candidate and businessman Donald Trump. Credit:AP It's not the first time spirited boos have been directed at Mr Trump on the debate stage. The same happened to him during a debate days before voting in New Hampshire, where he won the primary decisively on Tuesday. The fed-up, anti-establishment voters fond of Mr Trump are not regulars in the halls of debates sponsored by the Republican National Committee.

The exchange finally ended when moderator John Dickerson turned to former Ohio governor John Kasich, who expressed astonishment at what had just happened. "This is just crazy," he said. "This is just nuts. Jeez, oh man." The death of Scalia also brought urgency to Saturday night's debate, as candidates sought to position themselves as the party's best hope of preserving his unyielding conservative legacy. The loss of perhaps the most influential voice from the right on a court where conservatives held but a single-vote majority brought to the forefront of the bitterly contested race the issue of electability. With Mr Scalia's loss, conservatives no longer have a court serving as a bulwark against the liberal policies of the Obama administration. "His loss is tremendous," said Florida Senator Marco Rubio."He will go down as one of the greatest justices in the history of this republic."

The appointment of a Supreme Court justice is one of the most enduring legacies of any president, and it is likely to motivate the base voters in both parties who hold the greatest sway in presidential primary elections. Texas Senator Ted Cruz immediately reinforced his image as a conservative warrior, assuring the audience that he will lead the fight to resist any nominee President Barack Obama sends over to the Senate, demanding Mr Scalia's replacement gets chosen by the next president. "We are one justice away from a Supreme Court that will strike down every restriction on abortion," he said. "We are one justice away from a Supreme Court that would undermine religious liberty for millions of Americans." Mr Cruz is the only candidate who has served as a clerk on the court, and he established his political reputation arguing in front of it, as solicitor general of Texas, before he became a US senator. He dwelt on those experiences Saturday night, remarking that he knew Mr Scalia for 20 years. But as the candidates were pressed on whether Republicans stood on firm legal ground in pushing to delay the confirmation of a replacement until after Mr Obama has departed the White House, Mr Trump was predictably frank. He said he thinks Mr Obama will nominate someone "whether I like it or not. I think it is up to Mitch McConnell and the others to stop it," he said of the Senate's majority leader. "... It is called delay, delay, delay."

The infusion of Supreme Court politics into the debate comes as the freshly re-scrambled field campaigns in South Carolina, where several candidates are making what could be their final stand in the effort to take down Mr Trump. The field, once more than a dozen, is down to six, led by Mr Trump, who is a favourite to win South Carolina's primary next Saturday. Mr Trump has reclaimed his bravado after a disappointing second-place finish in Iowa's caucuses. His strength in early states and large lead in national polls has prompted panic in the Republican establishment, which is suspicious of his ideologically mixed bag of populist proposals and worried that his inflammatory rhetoric will alienate Latinos and other key constituencies in a general election. While the dynamics of the race have shifted considerably since the last GOP debate was held a week ago in New Hampshire, and the field has shrunk, no single candidate has yet been able to consolidate the anti-Trump vote. Mr Rubio, who had been on the rise after a strong finish in Iowa's caucuses at the beginning of the month, is now clinging to relevancy after a poor performance in last week's debate that many characterised as robotic and scripted. He has spent the last week apologising to supporters, promising to avoid a repeat, and, in a bit of bad luck, breaking a tooth on a Twix candy bar.

The Florida senator is fighting to repair his image as a fresh and articulate party spokesman, especially on foreign policy, where he has tried to establish himself as an aggressive voice. Mr Bush faces nearly as much pressure as Mr Rubio. He was desperate before New Hampshire, but finished just well enough to remain viable in South Carolina, where he is hoping to get a boost from his family's successes in the state. After distancing himself at times from the family name, Mr Bush is now clinging to it, with an event planned Monday evening in North Charleston with his brother, former President George W. Bush, who also recorded a radio ad on his behalf. TNS