On Monday night, the Republican Party was thrust off the edge of the political map into uncharted terrain, its leaders forced into the sort of moral crisis no American party has faced for decades. Its most popular presidential hopeful had gone far beyond the bounds of acceptable politics, core American values and basic human decency, by making a serious proposal to exclude an entire category of people from the United States based on their religious and ethnic identity.

The politics of racial exclusion, long banished from mainstream North American politics, had burst into the forefront of one of the world's largest and most influential political parties.

By Tuesday afternoon, it had become a problem for the entire United States. It was clear that Donald Trump's proposal to forbid people who identify themselves as Muslim from entering the United States, while quickly condemned by most politicians of the left, centre and right, had actually increased his popularity among Republican voters. A Bloomberg poll, taken on Tuesday, showed that of those who are likely to vote in the 2016 Republican primaries, 35 per cent approve of his proposal, and a similar number say they are therefore "more likely to vote for him."

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As pollster Frank Luntz, looking at similar numbers, concluded on Wednesday, "You actually have to consider the possibility of Trump winning the Republican nomination" – that is, if the party is unable to expel or marginalize him, something its leaders were wary of considering this week.

The United States is discovering, very quickly, that a charismatic single-issue candidate whose single issue is bigotry and intolerance can attract significant support, and possibly even seize control of a major party.

U.S. politicians from across the spectrum respond to Trump’s anti-Muslim comments 1:17

True, Republican candidates have certainly used racially divisive language and trafficked in conspiracy theories about Latinos and Muslims. In the 2012 primaries, at least three candidates repeated a conspiracy theory about a secret plot by ordinary Muslim Americans to impose Sharia law by stealth. And this year, some of Mr. Trump's competitors have offered less radical versions of his fearmongering (such as denying citizenship to undocumented Latinos, or accepting only those Syrian refugees who are Christian).

But no mainstream candidate in recent history has used discrimination and racial fear not as a means to power but as an end in itself, as a chief policy goal.

In other words, Americans have learned that they have something in common with most countries in Europe, where a third-place opposition party devoted almost entirely to opposing racial and religious minorities has become a growing fixture.

Mr. Trump's rise to political fame, his positions, his public style, his singular focus on immigration and ethnicity and, most importantly, his supporters are strikingly similar to those of Europe's latest extreme-right firebrands: Marine Le Pen of France's National Front, Nigel Farage of Britain's UK Independence Party, Jimmie Akesson of the Sweden Democrats, Heinz-Christian Strache of Austria's Freedom Party and Geert Wilders of PVV in the Netherlands (who heartily endorsed the Muslim-banning proposal of his near-lookalike Mr. Trump).

Most of these parties have been slowly growing in size and popularity as the big social-democratic and Christian-democratic parties have withered, and while their supporters are nowhere near a majority in any country, in some places – notably Austria and France – they are edging into second-place territory.

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Mr. Trump has plunged his country deep into this terrain, while simultaneously going further, with his Monday announcement, than almost any of these leaders have.

"The most amazing thing about Trump's latest statement, of not allowing any Muslims in, was that almost none of the European far-right parties would go that far," says Cas Mudde, a Dutch political scientist at the University of Georgia who analyzes anti-immigration political movements. "The only other party leader who's ever said that was Geert Wilders. I mean, Marine Le Pen hasn't made such statements, Strache hasn't – they've always separated Islamism from Muslims. Trump went, within two months, from being a little bit critical of Islam to being among the most Islamophobic politicians in the world."

This poses two big challenges. One is for the Republican Party, which now faces an existential crisis. And one is for the wider U.S. political system, which needs to figure out how a large proportion of voters – possibly 15 or 20 per cent – became so angry and alienated that they would support the darkest form of intolerance, and whether it can find a way to direct those voters' anxieties into more moderate politics.

Both challenges rely on a key question: Just who are these people supporting Donald Trump?

Campaign volunteer Andrea Nash, 21, of Hampton, N.H., wears a Trump campaign sticker while directing people to the overflow room before his speech at a rally at Winnacunnet High School. M. SCOTT BRAUER

Trump supporter Holly Couture stands in the Secretary of State’s office after he filed his candidacy in the New Hampshire primary. Ms. Couture said she was so excited at the chance to meet Mr. Trump that she couldn’t get to sleep until 1 a.m. the night before. She got his autograph on the campaign sign and tried to get a picture with him, but her camera failed. M. SCOTT BRAUER





THE DAWN OF THE DISAFFECTEDS

There is a genuine fear within Republican leadership and activist groups that Mr. Trump is going to tear the party in two. For he seems to have awakened a new group of voters whose aspirations are radically different from those of the Republican Party's traditional right wing.

"The phenomenon is real, and the danger Trump presents for the Republican Party is real," writes Ben Domenech, a Republican activist, publisher and former speechwriter in the administration of George W. Bush. "His statements have tapped into a widespread anger that has the potential to transform the Republican Party in significant ways. Ultimately, Trump presents a choice for the Republican Party about which path to follow: a path toward a coalition that is broad, classically liberal, and consistent with the party's history, or a path toward a coalition that is reduced to the narrow interests of identity politics for white people."

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That shift toward "identity politics for white people" has created a new kind of Republican voter.

During the summer, it was commonplace for Republican activists, columnists and insiders to say that Mr. Trump's support would vanish come primary time: The people who were supporting him did not seem to be Republican voters. In fact, the people supporting him didn't seem to be coming from anywhere the party recognized.

An ABC News/Washington Post poll in late July found that Mr. Trump's strongest support came from "people at the margins of the Republican primary process."

Notably, it found that his support is four times higher among those with no postsecondary education (32 per cent of whom support him) than it is among university graduates. Polling at the end of November showed similar results, with Mr. Trump the favoured choice among voters without college degrees by a two-to-one margin over other candidates.

Equally surprising: On issues other than race and ethnicity, Mr. Trump's supporters don't tend to be especially right-wing. The ABC/Washington Post poll found that among very conservative voters, 17 per cent supported Mr. Trump; among somewhat conservative voters, 24 per cent did; and among moderate-to-liberal voters, 27 per cent did.

Republican activists have frequently noted that the core Trump supporters aren't coming from the GOP base; rather, they're part of a "wild card" constituency, often alienated from mainstream politics, that votes Democratic or Republican – or, often, not at all. In its mammoth breakdown of American voting blocs, the Pew Research Center finds the group most likely to vote Trump is the community that it has variously labelled the "Disaffecteds" or, more recently, "Hard-Pressed Skeptics."

This group, representing between 9 and 13 per cent of the U.S. electorate, tends to be male (twice as many men as women today back Mr. Trump's politics), overwhelmingly white, and especially lacking in education: 7 in 10 of them have no postsecondary education, compared to 49 per cent nationwide.

As Pew's original, 2005 definition of the group read: "Disaffecteds are only moderate supporters of government welfare and assistance to the poor. Strongly oppose immigration as well as regulatory and environmental policies on the grounds that government is ineffective and such measures cost jobs." Eighty per cent of them said immigrants "are a burden on our country" nearly double the rate of the general American public.

They don't tend to be staunch partisans: In 2005, 68 per cent of the disaffected group were registered Independents, and only 32 per cent were Republican. They tended to vote for Mr. Bush over John Kerry in 2004, then for Barack Obama in 2008 and 2012. By 2014, this group, defined slightly differently, was half Democratic and 37-per-cent Republican. Now they've been captured by Mr. Trump.

Indeed, some observers say that the surprisingly strong turnout for overtly socialist Democratic Party candidate Bernie Sanders is coming from exactly the same constituency that's backing Mr. Trump. As the conservative writer Yuval Levin wrote in the National Review, "Both Trump and Sanders are calling attention to those political debates in which the inherent cosmopolitanism of modern capitalism is most deeply in tension with the inherent populism of modern democracy – especially, but by no means exclusively, immigration and trade."

It is perhaps easiest to understand the Disaffecteds as a case of failed integration. As the children and grandchildren of the old postwar U.S. white industrial working class, they have followed a trajectory, and fallen into ways of thinking, that are strikingly similar to those of some unsuccessful low-income immigrant groups in Europe: a low educational-attainment rate, lack of entrepreneurial success, reverse social mobility across generations, a tendency to self-segregate into ethnic enclaves and self-policed neighbourhoods, and, now, an increasing tendency to vote for extremist politics. They have not literally immigrated into America but rather have landed in a postindustrial, cosmopolitan economy, and resisted any effort to assimilate, instead choosing to turn against more successfully integrated American newcomers.

This alienated, white, generally male, older and undereducated constituency – perhaps 10 to 15 per cent of voters – has been driving the rise of new parties of the anti-minority right across the Western world.

Matthew Goodwin, a political scientist with Britain's Chatham House, has been analyzing the views and attitudes of people voting for anti-immigration parties across Europe and the United States. He says Mr. Trump's voters are strikingly similar to those backing far-right parties in the Netherlands, Britain or Austria.

"What's most important is the role of anxiety, nostalgia and fears of cultural change among white Americans – especially white Americans who have not gone to university and have felt financially cut adrift from the transformation of their economy and have felt under threat from rapidly rising rates of ethnic diversity," Mr. Goodwin says. "For these people, this identity politics has become more influential than traditional issues such as jobs and housing."

Those voters didn't so much walk away from mainstream politics as find themselves abandoned by it. The big parties of the centre-left and the centre-right saw educated elites as their crucial constituency, and aimed for aspirational groups such as young university graduates and suburban soccer moms, who increasingly drove election outcomes. The lost low-education white voters simmered in the background.

Then, in the years after 2001, anti-immigration activists and people peddling anti-Muslim conspiracy theories discovered this group.

"Immigration came up as a salient issue that cuts across that traditional party dividing line, and it became more important to working-class voters than the old left-right ideology," Dr. Goodwin says. "And the parties failed to respond to that quickly enough because they were fixated on middle-class university graduates who were numerous enough that they decided the fate of elections. And the old working class, who used to decide elections in the sixties and seventies, became smaller in size, increasingly disenchanted and open to the appeals of the radical right."

Donald Trump, after countless false attempts to run for president (he has launched bids in almost every election), finally stumbled upon this group, and its singular obsession with immigrants and minorities, and seemed instinctively to see that they were unclaimed in American politics. Some of them had fallen into the racially intolerant branch of the Tea Party movement; others had stopped voting. His angry appeal to ethnic fear and white identity politics galvanized them, and turned them into what is, at the moment, the Republican Party's biggest problem.

People listen as Mr. Trump speaks to supporters at a rally at the Weirs Beach Community Center in Laconia, N.H. M. SCOTT BRAUER

Trump supporters wait for his arrival outside at the New Hampshire State House to officially file his candidacy in the New Hampshire primary in Concord, N.H. M. SCOTT BRAUER

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THE REPUBLICAN SELF-IMMOLATION

For Reince Priebus, the former Tea Partier who chairs the Republican National Committee, this is the worst possible nightmare. After the dismal results of the 2012 presidential election, Mr. Priebus, along with most other Republican officials, recognized one of the new realities of U.S. political life: It is almost impossible to win the presidency if your party relies largely on the votes of white people. And in 2012, the Republicans received only 27 per cent of Hispanic votes and 6 per cent of African-American votes, a shortfall that analysts say cost them the presidency.

Mr. Priebus spent much of 2014 trying to raise his party's appeal to minority voters, boasting that he was spending about $8.5-million a month on outreach efforts. "What I'm saying is, instead of getting 6 per cent of the black vote in this country, if we get out there and fight and talk to people, then we get 15, then we get 20, and two years later we get 22 and 23. I'm in this for the long haul," he said in a television interview last year.

And then along came Donald Trump.

Bald eagle nips at Trump during photo shoot 0:55

In the wake of the real-estate mogul's sharp turn against Latin Americans – with his proposal a year ago to build a wall to keep immigrants out and to deport the 11 million full-time U.S. residents without citizenship – only 11 per cent of Hispanic Americans now support him. His African-American support was measured at 3 per cent in a recent CNN poll. And that's just the direct effect of Mr. Trump's candidacy. Republican officials fear that a wider revulsion caused by his foray into the fringes of ethnic-resentment politics will drive larger numbers of people – notably religious and racial minorities – away from their party.

An even deeper fear is that Mr. Trump, if successful, will redefine the Republicans as a party built around the politics of racial identity.

There are, as the Republicans have long known, a lot of people who will come out to vote for "identity politics for white people." But not anywhere close to a majority of Americans. The last time overtly racial-identity politics dominated the Republican Party, with the fire-breathing candidacy of Senator Barry Goldwater in 1964, the result was one of the most decisive defeats in presidential-election history, to Lyndon B. Johnson. Now, half a century later, there are a lot more minority voters, racial tolerance is vastly higher, and liberal Democrats are the fastest-growing political constituency.

That new reality was revealed starkly in an NBC-Wall Street Journal poll released on Thursday, which showed that 60 per cent of Americans have a "very negative" or "mostly negative" view of Mr. Trump (and only about 12 per cent were "very positive" about him) – and that almost 65 per cent have a "very favourable" or "mostly favourable" view of Muslims in their country.

The cheeky headline the Washington Post put on the poll, "Americans like Muslims more than they like Donald Trump," spells out the difficult decision faced by Mr. Priebus and his colleagues.

If Mr. Trump is allowed to prevail in the primaries, the Republicans' bid for the presidency is doomed – as, quite possibly, is the future of their party. But if they persuade him to leave the party or gaslight him out of the race – neither of which would be easy, as U.S. parties have no ability to oust members, and Mr. Trump is notoriously immune to influence or advice – then there's a good chance he will run as an independent, possibly taking his supporters and a good chunk of other Republicans with him. As Ross Perot showed in 1992 and Ralph Nader did in 2000, even a fairly minor third-party candidate can upset a presidential race.

Mr. Priebus dug himself deeper in August, when he forced all the party's candidates to sign a "loyalty pledge," in which they promise to support whichever candidate wins the primaries – and not to drop out and run a third-party campaign. Mr. Trump eagerly signed: It was intended to keep him from deserting, but it now means that the party is effectively stuck with him.

Real-estate mogul and Republican presidential candidate Donald Trump greets supporters after speaking at a rally at the Weirs Beach Community Center in Laconia, N.H. M. SCOTT BRAUER

Faced with this dismal pair of options, the Republican leadership appears to have frozen in terror. When Mr. Priebus finally got around to responding to Mr. Trump's noxious proposal this week, his words were mild and passive. "I don't agree," he said, "We need to aggressively take on radical Islamic terorrism but not at the expense of our American values."

And then, nothing. In the face of the greatest threat to its political survival, the party appears to have chosen to bide its time, and hope that Mr. Trump will finally do or say something that will diminish, rather than bolster, his popularity.

Mr. Trump is not going to become President of the United States. That prospect, in any imaginable scenario, would be utterly undone by the very things that make him appeal to a certain angry, ignored, white group of voters. His popularity among these excluded men is exactly what will prevent the much larger, more multihued, more moderate mainstream American electorate from considering him a viable presidential candidate.

But Mr. Trump will have a profound effect on American politics. That, after Monday night, is already taking place. Suddenly, that disaffected 10 or 15 per cent of voters are no longer a group to be ignored or addressed in coded dog-whistle appeals: It is a growing threat, and, for some, an opportunity.

Both moderate Republicans and Democrats are now searching for a new language with which to speak to this group – to appeal to its sense of exclusion and anxiety, with real economic solutions rather than myths of brown-skinned invaders – in hopes that their parties will never get trumped again.

Doug Saunders is The Globe and Mail's international affairs columnist.