It's a common refrain that Donald Trump rallies are not exactly a mirror image of the diversity of the American population. His pitch to black voters has often been offered in fron of audiences that are nearly entirely white, in towns and cities that are similarly homogeneous. At a glance, Trump seems to have the clear support of white Americans. But, compared to past Republican candidates, that's not necessarily the case. His large audiences may be mostly white, but whites are much more split than they were four years ago.

White Americans have voted for the Republican over the Democrat in presidential elections every single year since at least 1972, according to exit polling.

In 1992 the margin was closer than at any time before or since. But as the electorate has grown more diverse in recent elections, the Republican candidate for president has earned more support from white voters by at least a double-digit margin. In 2012, the margin was 20 points for Mitt Romney over Barack Obama. White men, white women, whites with and without college degrees all preferred Romney's candidacy.

That's not the case now. Trump still has a lead of 16 points with likely white voters in a four-way race, but the white vote is much more splintered than it was four years ago. White women and college-educated whites of both genders are less committed to Trump than they were Romney, and, according to the new Washington Post-ABC News poll released on Sunday, white women, whites with college degrees and white women with degrees (groups with some overlap, of course) all prefer Hillary Clinton to Trump.

The change since 2012 is often stark.

Trump has 28 points more support from white men without a college degree, leading Clinton with that group by nearly 60 points. But white men with college degrees and white women are more likely to back Clinton -- with white women with college degrees even more likely to support the Democrat this year than white men without degrees are to support the Republican. They support Clinton by a 25-point margin, and are the reason that white women and white college graduates overall prefer her.

Every one of those 2012 bars is on the Republican side of the graph. This year, that's not the case. The reason that Republicans fared worse with whites in 1992 and 1996 wasn't only because of Ross Perot: White women split in 1992 and backed Bill Clinton four years later.

When we asked poll respondents how certain they were to vote, some other interesting patterns emerged. College-educated whites were more likely to say they planned to vote, which is not surprising. There's a consistent link between income and education and likelihood to vote. But white men, particularly white men with college degrees, were less likely now to say they were certain to vote than in September 2012.

One possible reason for that apathy may be that the unpopularity of both Trump and Clinton is inspiring some regular voters to sit this one out.

It's remarkable that, despite the split in the white vote, Trump's not doing much worse than Romney in polling. White voters are expected to be a smaller percentage of the electorate than they were four years ago, according to Pew Research, meaning that a collapse of support from white women and college educated whites could doom Trump. But his gains with white men without degrees seems to be offsetting it. If Donald Trump wins, it's because they came out to vote. If he doesn't, it's in part because his candidacy split a reliable Republican constituency in a way that Mitt Romney didn't.