George W. Bush won a bruising South Carolina presidential primary on his way to the Oval Office, as his father did before him.

Now it's his brother's turn, and for Jeb Bush, the most consequential foreign policy decisions of his brother's time in office are suddenly front-and-centre of his bid to keep alive his campaign for the 2016 Republican presidential nomination - thanks to businessman Donald Trump.

The former President had already announced plans to campaign for his younger brother this afternoon in South Carolina, marking his most direct entry into the 2016 race to date. Then Trump, the Republican front-runner, used the final debate before the state's primary as an opportunity to excoriate George W. Bush's performance as commander-in-chief.

The former President, Trump said, ignored "the advice of his CIA" and "destabilised the Middle East" by invading Iraq on dubious claims that Saddam Hussein had weapons of mass destruction. "I want to tell you: they lied," Trump said. "They said there were weapons of mass destruction, there were none. And they knew there were none."


Trump didn't let up as Bush tried to defend his brother, dismissing his suggestion that George W. Bush built a "security apparatus to keep us safe" after the 9/11 attacks. "The World Trade Centre came down during your brother's reign, remember that. That's not keeping us safe."

The onslaught - which Jeb Bush called Trump enjoying "blood sport" - was the latest example of the tycoon's penchant for mocking his rival as a weak, privileged tool of the Republican Party establishment, special interests and well-heeled donors.

But the exchange also highlighted the former Florida Governor's embrace of his family name and history as he jockeys with Florida Senator Marco Rubio and Ohio Governor John Kasich to emerge from South Carolina as the clear challenger to Trump and Texas Senator Ted Cruz. The approach tacks away from Bush's months-long insistence that he's running as "my own man," but could be a perfect fit for South Carolina. Noted South Carolina Senator Lindsey Graham: "The Bush name is golden in my state".

George W. Bush retains wide appeal among Republicans. Attacking him carries risk for Trump, given the Bush family's social and political ties in South Carolina and the state's hawkish national security bent, bolstered by more than a half-dozen military installations and a sizable population of veterans.

The foreign policy fracas is only the latest row among 2016 candidates over many of the basic tenets that have guided Republican and conservative thinking since the Reagan years, from free trade to the extent to which the federal government should be involved in providing health care for its poorest citizens.

The increasingly harsh discussions amount to an existential crisis within the Republican Party and reflect the growing influence of non-ideological, populist voters who have flocked in particular to Trump's nationalist "Make America Great Again" message.

Trump was unapologetic, saying that he is a truth-teller and that the majority of Americans - weary of war, alienated by the political class and thirsting for a populist revival - would heed his call. "The war in Iraq has been a disaster," Trump told CBS. "It started the chain of events that leads now to the migration, maybe the destruction of Europe. (Bush) started the war in Iraq. Am I supposed to be a big fan?"

Todd Harris, a senior adviser to Rubio, echoed the sentiment of many in the GOP when he said after the debate that Trump "was at war with the Republican Party".

So far, at least, it is a war that many Republicans are willing to wage alongside Trump. A new poll released by CBS News showed Trump surging here ahead of Sunday's South Carolina primary.

The survey showed Trump with the backing of 42 per cent of Republican voters.

The poll was taken before the debate and the ensuing fallout, which many Republicans predicted would limit Trump's appeal going forward.

Nevertheless, the coming weeks will test whether the ties that have bound the GOP for a generation will unravel entirely.

Graham said of Trump: "This man accused George W. Bush of being a liar and suggested he should be impeached. This man embraces (Russian President Vladimir) Putin as a friend. The market in the Republican primary for people who believe that Putin's a good guy and W. is a liar is pretty damn small."

As confident as the Republican establishment is that voters will eventually turn against Trump for his apostasies and controversies, there is little evidence that they will.

The escalating quarrelling may increase the likelihood of a long, expensive and potentially futile effort to unite Republicans around the eventual nominee.

The barbs at Sunday's debate were ferocious and personal: Trump made fun of Bush's mother and bickered with him over whether Bush had threatened to drop his pants and moon people, which he had; Rubio jabbed Cruz for not being fluent in Spanish; and they all seemed to call one another liars.

Pollster Frank Luntz, who for years has helped Republicans carefully calibrate their language to appeal to a broad range of voters, was aghast.

"If 10-year-old kids spoke to their teachers the way those candidates spoke to each other, those kids would be suspended."

The Bush family in South Carolina

• In 2000, George W. Bush beat John McCain in a nasty contest, marred by rumours that McCain had an illegitimate black child. McCain adopted a child from Bangladesh.

• Exit polls showed George W. Bush won nearly every demographic group.

• George H.W. Bush, the 41st President, won twice here, beating Bob Dole in 1988 and Pat Buchanan in 1992.

• One of the elder Bush's top strategists, Lee Atwater, hailed from South Carolina

- AP, Washington Post - Bloomberg