The political science shows that who's ahead at this point in time stays ahead - always! Almost always, anyway.
Here's the short of it from the HuffPost Pollster email report.
Historical precedent suggests that the current polling has predictive power. Donald Trump doesn’t seem to be ginning up GOP enthusiasm. And both candidates’ popularity bounces are starting to fade. This is HuffPollster for Friday, August 12, 2016.
CANDIDATES LEADING AT THIS POINT USUALLY WIN THE ELECTION - Andrew Prokop: “We are two weeks out from the final day of the Democratic convention. And according to historical research by political scientists Robert Erikson and Christopher Wlezien, this is usually the time that the convention fog finally lifts and the polls become more predictive. ‘Although the convention season is the time for multiple bounces in the polls, one party ends up with an advantage when the dust clears. And this gain is a net convention bump rather than a bounce,’ Erikson and Wlezien write in their book The Timeline of Presidential Elections. That is, once the volatility dies down, one candidate usually emerges from the convention chaos with a durable lead. And almost always, the political scientists find, that candidate goes on to win the election….Indeed, the authors looked at general election contests going back to 1952, and found that the candidate who was in the lead two weeks after the conventions ended went on to win the popular vote every single time….[C]onventions tend to have a consistent and profoundly important impact of the type that’s hardly ever observed at any other brief phase of the campaign, even those much-hyped fall debates.” Vox
You can get the details (numbers! charts!) here in the Vox.com report "Presidential candidates leading polls at this point in the campaign have almost always won." The Vox report winds up this way.
... And there’s always the possibility, however small, for some historically unusual major news event — a scandal, a terror attack, an economic crisis — to scramble things in a way we don’t usually see.
Still, Erikson and Wlezien’s research shows that conventions tend to have a consistent and profoundly important impact of the type that’s hardly ever observed at any other brief phase of the campaign, even those much-hyped fall debates.
"Once the conventions are over, further campaign events — even presidential debates— rarely result in dramatic change," they write in a 2012 update to their book. And if Hillary Clinton’s current lead holds up in the next few days of polls, that will be comforting news for her and Democrats.