The polling benchmarks Trump needs to meet in order to pass Hillary

The graph also shows that in past elections, polls typically don’t move more than four points after late September or early October (about 30 or 40 days before the Election). It’s pretty easy to see this in the graphic — just compare the density of points under the trend line to those above it. So if Trump wants to be in striking distance of Clinton, his best bet is to close the gap to three or four points before October.

It’s important to note this isn’t an ironclad rule. If this election follows the recent trend — a polarized electorate with relatively few swing voters — then it will be difficult for Trump to move the polls more than a couple points. But if this election follows an earlier historical pattern — with less polarization and more voters remaining undecided — he may be able to make up more ground later.

Once October hits, Trump would need to move opinion quickly in order to win the race. Roughly three weeks out from Election Day, the polls start to converge reasonably quickly on their final levels. This makes sense intuitively — Trump and Clinton will have already debated once and many voters will have learned about both candidates and made their final decision. So that final downward dip shows that undecided voters don’t always wait until Election Day — many of them settle on a candidate before or during the month of October.