Trump has continually tested the limits of what a candidate can say and do with apparent political impunity.

Donald Trump continues to go where no recent candidate for president has gone before, plunging the Republican Party and America into another round in the tumultuous debate about immigration, national identity, terrorism and the limits of tolerance.

Trump's call for a ban on Muslims entering the United States marked a sudden and sizable escalation and in this case one that sent shockwaves around the world in the inflammatory and sometimes demagogic rhetoric of the candidate who continues to lead virtually every national and state poll testing whom Republicans favour for their presidential nomination.

Nothing in modern politics equates with the rhetoric now coming from Candidate Trump. There are no perfect analogies. One must scroll back decades for echoes, however imperfect, of what he is saying, from the populist and racially based appeals of then-Alabama Governor George Wallace in 1968 and 1972 to the anti-Semitic diatribes of the radio preacher Charles Coughlin during the 1930s.

Historian David Kennedy of Stanford University said there are few comparisons, adding that, in branding an entire religious class of people as not welcome, Trump "is further out there than almost anyone in the annals of [US] history".

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From the day he announced his candidacy in June, Trump has continually tested the limits of what a candidate can say and do with apparent political impunity. In that sense, he has played by a different set of rules. In the wake of his latest provocation, the question arises again: Will this finally stop him? Everything to date suggests that those who think it will should be tentative in their predictions.

Those already drawn to Trump have shown remarkable willingness to accept the worst and continue to support him. In reality, it will be another 60 days or more for any definitive answers to emerge.

Only when voters begin to make their decisions in the caucuses and primaries that begin in February will the final verdict be delivered on the size and strength of the movement that has rallied behind him.

"This is a new campaign for a new century in which viral populism, most conspicuous on the GOP side, is the engine of our politics," Ross Baker of Rutgers University noted the day before Trump's latest outburst.

"Trump, above all others, has sensed this and is profiting from it. His reading of the anger and anxiety of the GOP primary electorate is positively seismographic. He senses what's eating at people and, in his own bizarre way, is most attuned to the electorate of any of the hopefuls."

Even as Trump yesterday sought to soften slightly what he had said on Tuesday, the condemnations mounted. He drew rebukes across the globe, from the leaders of two of America's most important allies, Britain and France, to Syrian refugees in Lebanon's Bekaa Valley. Rarely has a presidential candidate generated such alarm abroad.

At home, the condemnations were just as swift and nearly universal. The harshest came from Martin O'Malley, the former Maryland Governor seeking the Democratic nomination, who tweeted on Tuesday that Trump "removes all doubt he is running for president as a fascist demagogue".

Hillary Clinton sent out a tweet yesterday that said in big letters, "Love trumps hate". White House press secretary Josh Earnest said what Trump advocated should disqualify him from ever serving as president, and Earnest added that Republican candidates who refuse to say they would not support Trump as the party's nominee also are disqualifying themselves.

Republican leaders stopped short of that.

But many who in the past have seemed hesitant to tangle with the master of the political counterpunch, were quick to state their disagreement. Former Governor Jeb Bush said Trump had become "unhinged".

House Speaker Paul Ryan said this was not what the party stands for. Former Vice-President Dick Cheney said it "goes against everything we stand for".

Others were more tentative. Senator Ted Cruz, who said he did not agree with the proposal about blocking Muslims from coming into the country, also commended Trump for focusing attention on the need to secure the borders of the US.

Former Pennsylvania Senator Rick Santorum said there was a more practical way to accomplish the same goal.

But as the political establishment rushed to criticise Trump, there is little doubt that he has tapped into a strain of anti-immigrant and anti-foreigner sentiment that has ebbed and flowed through American history.

Donald Trump autographs a supporter in Virginia. Photo / AP
Donald Trump autographs a supporter in Virginia. Photo / AP

There are a number of antecedents over the past century that put Trump's candidacy and the responses to it into historical context.

After World War I, a wave of immigration from Europe to the United States, coupled with fears of the spread of worldwide communism after the Bolshevik revolution, led to strikes, riots, violence, anarchism and ultimately a powerful backlash against immigrants.

Then-Attorney General Mitchell Palmer led a series of infamous raids, rounding up suspected radicals and trying to deport them.

A rising nationalist and nativist strain fuelled by the war and its aftermath eventually led Congress twice in three years in the early 1920s to enact strict new quotas on immigration, sharply limiting the influx of those fleeing a continent devastated by the war for opportunities in the America.

In the past decade, illegal immigration from Latin America has repeatedly emerged as a hot-button issue of US politics, playing out most prominently inside the Republican Party. Efforts to pass comprehensive immigration reform, advocated by then-President George W.

Bush during his second term in office, faltered because of conservative resistance. Renewed efforts under President Barack Obama met similar opposition from conservatives in and out of Congress.

Fears of terrorism have now been layered on top of the issue of illegal immigration. Long-standing fears about Isis (Islamic State) terrorism have intensified since the attacks in Paris and San Bernardino.

Republican presidential candidate Donald Trump energizes the crowd during a campaign rally at Dorton Arena in Raleigh. Photo / AP
Republican presidential candidate Donald Trump energizes the crowd during a campaign rally at Dorton Arena in Raleigh. Photo / AP

Stanford's Kennedy pointed to "inchoate, diffuse, free floating anxiety" brought on by economic strains, the nation's inability to extract itself from Middle East wars and a generally unsettled world as other causes for Trump's appeal.

Kennedy also noted that in contrast to times past, what once held extreme expressions in check no longer does.

"We've known for a long time that we're just less trustful as a people. We have less confidence in our major institutions and our leaders.

He gets denounced routinely when he does these things, and everyone gets up and says this is not a voice we should listen to. But nobody has credibility on the other side. Nobody has the cultural authority to put this guy down. All the condemnation in the world falls on deaf ears."

What once might have seemed inconceivable in political debate has become acceptable, at least to a part of the population. That makes this moment a potential inflection point in the life of the country.

Those already drawn to Donald Trump have shown remarkable willingness to accept the worst and continue to support him. Photo / AP
Those already drawn to Donald Trump have shown remarkable willingness to accept the worst and continue to support him. Photo / AP

For the Republican Party, it highlights what has emerged as a deep split between the party elites and at least a portion of the rank and file.

Next week's debate in Las Vegas, the final GOP debate this year, will bring the candidates together in what has become the most virulent moment of the campaign. Trump will be under fire, but he has been there before and survived, even prospered. Will this moment prove any different?

Beyond that, Trump has brought into sharper focus important questions that will play out during the coming election year: What can be done to make Americans feel safer? What will impede or encourage recruitment by Isis? What does it mean to be an American?

What kind of image does this nation project around the world? Along with the rhetoric of Donald Trump, the stakes for 2016 have escalated dramatically.

- Bloomberg