When the candidate mocked a reporter with a disability last November.
Democrats have made sure the public has seen and heard about Trump's extended riff over and over again. It has been played in ads created by the Clinton campaign, including a highly-circulated one involving children silently watching, and by groups working on Clinton's behalf.
Bill Clinton even said in his convention speech that his wife "never made fun of people with disabilities. She tried to empower them based on their ability."
And Clinton surrogate Tom Harkin, a prominent voice on disability policy, said recently, "Democrats believe in working together and bringing people with disabilities in to develop policy. Donald Trump? He makes fun of people with disabilities. That's a throwback to a half a century ago."
Trump has denied that he meant to mock the reporter, Serge Kovaleski, who has arthrogryposis, a congenital condition affecting the joints.
His initial offense, in Trump's eyes, was denying the candidate's description of Kovaleski's reporting on unsubstantiated allegations that Muslims in New Jersey celebrated on 9/11.
As recently as July 29, Trump insisted, "I didn't know what he looked like. I didn't know he was disabled."
He has maintained that his physical affect in imitation of Kovaleski was because "He was groveling, grovel, grovel, grovel. That was the end of it. All of a sudden, I get reports that I was imitating a reporter who was handicapped. I would never do that."
The Washington Post's factchecker responded, "Much of what Trump says is Four-Pinocchios false."
Among other falsehoods described by the Post, including the fact that the "groveling" charge makes no sense in context, and Kovaleski, who now works for the New York Times, has said, "Donald and I were on a first-name basis for years."
The poll was conducted before Trump sparked furor with a comment about "Second Amendment people" that was interpreted by many as a veiled suggestion of violence against Clinton.
Still, given the lasting stigma around people with disabilities and the easiness with which pop culture has mocked them over the years, the outraged reaction from the public is striking.
"People are starting to see people with disabilities for their abilities," said Jennifer Laszlo Mizrahi, president of the advocacy group RespectAbility, citing Olympic swimmer Michael Phelps, who has Attention Deficit Disorder, and Nyle DiMarco, winner of America's Next Top Model and Dancing with the Stars, who is deaf.
"But one thing they don't want to see about people with disabilities is for us to be bullied," she added.
"The reaction to it, that so many people say it's the worst thing he's done, is indicative that we've turned a corner, where it's just not socially acceptable the way it was," said writer and activist Andrew Pulrang.
Still, he said he found himself wondering, "Why is this the thing that is worse than all the other groups than he's targeted?" One possible answer: "We're easy to feel protective towards. Part of ableism is a heavy dose of paternalism."
Pulrang hopes the outrage translates into more interest in policies that affect people with disability. "There are other things more important than hurting our feelings," he said. "We have no idea if Trump has any ideas about disability policy."
Though several Republican primary candidates filled out a candidate questionnaire produced by Mizrahi's group, Trump has yet to do so.
"Meanwhile, Hillary Clinton filled it out in full, and spoke about disability issues at the Democratic convention, as did people with disabilities themselves," Mizrahi said.
Some of the Democrats' attempts to draw a contrast with Trump around people with disabilities have drawn mixed feelings from advocates.
For example, the Democratic SuperPAC Priorities USA created an ad featuring the parents of a child with spina bifida. The father's final words are, "When I saw Donald Trump mock someone with a disability, it showed me his soul. It showed me his heart. And I didn't like what I saw."
The ad drew some criticism from the disability rights community.
"It follows a pattern that we're very used to, which is that it's on our behalf," said Pulrang. "We didn't hear from the disabled daughter, who seemed from the ad to conceivably be able to have an opinion of her own. Almost as if the real problem is that the parents were offended."
Still, Pulrang added, a few weeks later, Priorities USA came out with another ad that did feature a person with a disability speaking directly, which many activists appreciated.
For people who are used to condescension or outright erasure, Trump's behavior carries a certain irony.
"Inclusivity for people with disabilities is now a matter of presidential politics," wrote David Perry at the Atlantic. "That likely wasn't Trump's intention when he mocked that reporter months ago. But it's a satisfying result."