Can Donald Trump catch up to Hillary Clinton in the polls before Election Day?

I’ve asked that question a few times over the past couple months, and for much of August and September the answer has been a qualified yes. But recent events -- a bad performance at the first presidential debate, the release of an interview showing Trump making lewd comments about women, and a middling second debate performance, to name a few -- have widened Clinton’s lead. And based on quantitative polling benchmarks I developed in August, a Trump comeback is improbable.

This graphic summarizes how much polls typically move between Election Day and a given number of days beforehand. The green points show the difference between polls on a given day and the final polling average heading into the election for all presidential races from 1972 to 2012. The black line is the trend line -- it provides an upper limit for how much polls typically move between a given day in the fall and Election Day. (Details on the math and data can be found here.)

Two specific features of this data jump out: Trump is missing his benchmarks and his ability to shift the polls may quickly start to wane.

Trump Is Missing His Benchmarks

Past polling data shows that three weeks out from the election, polls usually don’t move more than 3.4 points toward either candidate. Clinton leads Trump by 5.5 points in the RealClearPolitics two-way national average and by 5.7 points in the four-way national average (which includes third-party candidates Gary Johnson and Jill Stein). Even if Trump moved the polls by about three points in his direction (which isn’t guaranteed, since Clinton could add about three points to her margin) Clinton would still have a two- to three-point national lead heading into Election Day. In that case, the smart money would be on her.

Clinton has led by larger margins at earlier points in the campaign. For example, she led Trump by more than seven points in the RCP two-way average in the aftermath of the Democratic National Convention. At that time she was enjoying a convention bounce and Trump was losing ground after a public feud with the Khan family, who lost a son in Iraq. Clinton also led Trump by double digits in March 2016 (when the GOP primary was still hotly contested) and the summer of 2015 (at the beginning of the Trump phenomenon). At those junctures, Trump still had ample time to change the polls. But three weeks is a much shorter window than three, eight or 18 months. And as Harry Enten of FiveThirtyEight notes, there is no clear historical precedent for a Trump comeback at this point.

It’s important to note that these benchmarks aren’t ironclad laws. Something unexpected could happen (e.g. Trump could shake off the new sexual assault allegations, have a great final debate performance and find some new damaging piece of information related to Clinton’s emails) that allows the Republican to make up ground faster than the majority of his predecessors. But Trump win scenarios often involve nearly every significant event over the next three weeks breaking his way, which is possible but not the most likely outcome.

Another Problem for Trump: The Cement Is Drying

These benchmarks show another troubling sign for the GOP nominee. Over the next few weeks, his ability to shift the polls toward him will likely decrease quickly. This is apparent in the graph above. About three weeks out from the election, the trend line starts to dip quickly. Between now and next Tuesday it will move from 3.4 points to three (drops like that usually take two or more weeks). And in the final week of an election, national polls usually don’t move more than two points. In other words, Trump’s ability to make up ground will decrease as the days tick down.

That should make Clinton feel more secure than she did last month. Three weeks ago, she had a roughly four-point lead in the four-way RCP average. At that time, my benchmarks showed that the polls could shift by about 4.3 points before Election Day. That kept Clinton from running out the clock -- she needed to stay engaged, campaign hard and not give Trump too many opportunities to grow his support.

But that drop in the trend line could allow Clinton to play “prevent defense” for the next three weeks. Even if Trump managed to shave a couple points off of Clinton's margin, she would still have a comfortable lead heading into Election Day. And every day that Clinton manages to keep her current margin is a day in which Trump’s ability to shift the race diminishes.