In the RealClearPolitics polling average, Donald Trump has had a lead for four days since June 1. That was during the Democratic convention, when Trump had the advantage of his own poll bump but before Clinton got hers.

The reason Clinton keeps leading is that, while neither candidate usually gets a majority of support, Trump has only rarely matched or exceeded her support in polling. Only on four days since June has Trump been at or above 45 percent in the RealClearPolitics average nationally. Clinton's hit that mark on 85 days. She keeps staying just ahead of him.

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You've probably heard us (and anyone else who opines on the election) explain why: Donald Trump has a core base of support that he's having trouble expanding. But who's in that base?

We can look at that by dipping into Post-ABC polling since December of last year. If we look at all respondents -- a group that is broader than both registered or likely voters, we can see that Trump's never had the support of less than 36 percent of Americans. In every other poll, he's been within a 6-point range above that; in our most recent survey, released at the end of last month, Trump's at 42 percent with all Americans. Clinton's core base of support is a minority of the population, too: She's always had at least 48 percent. But the range of movement in polls has taken her as high as 53 percent. In that last poll, she was at 49 percent.

Why does this poll show Clinton with a 7-point lead? Because this is everybody. Narrow it down to likely voters, and Clinton's lead was 2 points. Clinton's base -- younger voters, nonwhite voters, have historically been less likely to vote, so looking only at the people likely to vote boosts Trump's numbers. Since the question we're trying to answer is who's in Trump's base of support, though, we'll keep looking at everyone overall.

A few of the assumptions you might have made about Trump's core base of support are correct. Trump's never been lower than 42 percent with men, but never been that high with women.

Trump's never been had less than 48 percent support from white voters, well above Clinton's peak with that group. But he's maxed out at a little over 20 percent with nonwhite voters.

So Trump's core of support is heavily white men. And, more broadly, white men without a college degree. He's never had the support of less than 62 percent of that group. He does well with white men with college degrees and with white women without degrees, too, but with those two groups, he's a bit closer to Clinton. Clinton's never had less than 47 percent of the vote from white women with college degrees, while Trump's peaked at 41 percent.

White men without college degrees, and, more broadly, white voters who aren't women with degrees. That's been the unwavering core of his support.

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The perpetual question is where Trump can expand that core outward. You can see from the charts above that his current polling is the highest its been with women and nonwhite voters. Theoretically, those figures could keep moving up -- but that seems much less likely, given the fall out from the hot-mic recording of Donald Trump.