You can see the pattern since the middle of June in the RealClearPolitics polling average. Clinton's now got a fairly big lead, thanks to Trump's support dropping off.
That's how Trump's campaign has gone. He's got a core of support, made up heavily of white men without college degrees, and has at times managed to expand that outward. But those expansions tend to then contract as Clinton has a good convention or debate or Trump is caught on tape casually talking about sexual assaults. That sort of thing.
The net result is that Trump is constantly running behind. In the RealClearPolitics average, Trump's been in the 41- to 41.9-percent range more than any other in a head-to-head contest. Clinton's been at 45 to 45.9 percent most regularly. In a four-way contest, those percentages shift downward a bit, but the point is the same: Clinton stays just ahead.
On average, Clinton's been at 46.1 percent in the RealClearPolitics average since June 15. Trump's been at 41.9 percent. In 146 polls tracked by Huffington Post Pollster over that period, Clinton's been under Trump's average only 15 times. Trump has been over Clinton's only nine times.
Most of Trump's over-average polls have come recently, thanks in part to that same undecideds-deciding effect. As the old saying goes, he needs to poll higher than Clinton on only one day: Nov. 8.
The challenge is that he has done consistently better than her for only one period since mid-June, the days after the Republican convention and before the Democratic one. It's that little red blip on the first graph. Those days were the apex of Trump's messaging: four days of pro-Trump rhetoric on the news, framed in the light the campaign felt was best.
And at the end of it, he was basically tied.