For Republicans, there are the scary numbers – the polls that suggest Donald Trump will suffer a humiliating defeat to Hillary Clinton, possibly surrendering states Democrats haven't won in decades, maybe dragging a lot of GOP legislators down with him.

And then there are the terrifying numbers, which should be keeping them up at night because they can't be blamed on their nominee alone, and could remain dire long after his grip on their party has been released.

Demographic trends, not Donald Trump, are what pose the biggest existential threat to them.

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As the U.S. population rapidly changes, consistently becoming more diverse and more educated, the Republicans are increasingly beholden to one shrinking segment of it drowning in toxic nostalgia: white people, especially white men, who don't have college degrees.

It's a problem party leaders know well. As testament to that, there is the remarkable postmortem commissioned by the Republican National Committee after Mitt Romney's loss to Barack Obama in 2012 – the one that called for an embrace of "comprehensive immigration reform" and other policies that would signal to minority communities that the GOP wanted their support.

But, as they've discovered since then, with Mr. Trump accelerating rather than halting the Republicans' over-reliance on their base, they spent a half-century creating a monster they can't easily get back in its cage.

Starting in the 1960s, the party of Lincoln – the party that under Dwight Eisenhower in the 1950s had been advancing civil rights for African-Americans – began a concerted effort to supplant the Democrats as the party of relatively traditionalist whites.

You can trace it back to Barry Goldwater's 1964 candidacy in direct opposition to the Civil Rights Act, although that was as much visceral as strategic – an uprising against party elites led by a candidate too kooky for the electorate's taste. A better election to start with might be 1968's, when Richard Nixon's more refined messaging about "states' rights" and law and order played to voters fearful about the era's tumult supplanting a social order that had served them well.

You can follow it through the 1980s, in Ronald Reagan's successful courting of "Reagan Democrats" – blue-collar whites who believed their previous party of choice had become too socially liberal, too wedded to policies like affirmative action, too much a coalition of minorities and coastal elites – with optimistic messaging about returning to the values of yore. Before that decade was out, George H.W. Bush benefited from some of the most notorious racial dog-whistling in U.S. history – notably the "Willie Horton" ad, playing to white voters' fear of black criminals – in a comeback win over Democrat Michael Dukakis.

You can pick it up again, more recently, in the populist anger of Sarah Palin running alongside John McCain in 2008, and the subsequent stoking of suspicions about the otherness of the country's first black president. And you can see immigration fears having entered the equation, as the country's Hispanic population has skyrocketed, to the extent that the relatively moderate Mitt Romney talked about the merits of "self-deportation" for those without status while seeking the presidency four years ago.

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It has not been a completely straight line: George W. Bush, for instance, made a concerted effort to win over Hispanics, and eschewed anti-Muslim rhetoric (of the sort seen of late) after 9/11. But until recently, at least, you could see in Republicans a confidence that their party had wound up on the right side of identity politics.

And they were right about that, until they weren't.

Demographics were not the sole reason they won five of six presidential elections from Mr. Nixon's first win to the elder Mr. Bush's lone one, but they sure helped.

And demographics are not the sole reason they have lost the popular vote in five of the six elections since, with a very good chance of going six for seven, but they are certainly a very big hindrance.

As recently as 1980, exit polls showed, only 12 per cent of voters were non-white. By 2012, that share had jumped to 28 per cent. And as it continues to grow, particularly because of growing Hispanic and Asian-American populations as well as improved turnout, those voters have turned away from the Republicans more than ever.

But it's not just about ethnicity; it's also about education.

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One of Mr. Reagan's great victories, particularly in his 1984 steamrolling of Walter Mondale, was to complete the shift of whites without college degrees – key to every Democratic victory to that point – over to the Republicans. The downside was that Republicans saw their once-mighty margins among college-educated whites start shrinking.

At the time, that was a pretty good tradeoff, since non-college whites accounted for more than 60 per cent of all voters. But by 2012, exit polls showed them down to just 34 per cent – and they're expected to continue falling by two or three percentage points every four years.

Now, running against Mr. Trump, the Democrats appear poised to win college-educated whites – a population growing alongside (if not as quickly as) minority voters – for the first time.

In other words, the Republicans may be about to find themselves overwhelmingly reliant on a shrinking segment of the population that is too small to win with. But just as problematic, and more easily overlooked in the heat of this campaign, is that that segment may still be too big from which to easily move on.

In theory, if Mr. Trump loses, the Republicans could easily choose a candidate next time around who broadens their appeal.

In practice, there may again be a major obstacle to that.

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That one demographic they have successfully rallied to their corner will still be big enough to overwhelm other voters in their primaries. And if the base rejected the post-2012 elite consensus, in the form of the postmortem that has already become a quaint artifact, there is no reason to believe it will suddenly have a come-to-Jesus moment after 2016. If anything, Mr. Trump's willingness to claim the election is rigged against him could harden resolve – leading to a relatively moderate nominee having to pander in ways unpalatable to the wider electorate, the way Mr. Romney did, or else someone like Mr. Trump who may be unpalatable from the outset.

About now, that gamble the Republicans took back in the halcyon days of the 1960s and 70s and 80s looks less winning. And that 34 per cent, or whatever smaller figure blue-collar whites now account for, looks like the most frightening number of all.