Sullenberger, whose landing of the plane in the New York river has been dubbed the “Miracle on the Hudson,” in part because everyone aboard survived, referred to remarks made on Thursday by Lara Trump, wife of Eric Trump, the president’s son. As The Hill reported, she seemed to mock Biden’s stutter at an event in Washington, D.C. that was held to promote her father-in-law’s reelection campaign.

“I feel kind of sad for Biden,” she said. “… I’m supposed to want him to fail at every turn, but every time he comes onstage or they turn to him, I’m like, ‘Joe can you get it out? Let’s get the words out Joe.’”

Vox journalist Aaron Rupar shared a clip of Trump’s remarks on Twitter.

In his opinion piece for the Times, Sullenberger wrote that he, like Biden, has had to deal with a stuttering problem. He wrote that he could still recollect the anxiety of having difficulty speaking when he was a boy.

“I remember vividly the anguish of being called on in grade school, knowing that I was going to have a hard time getting the words out; that my words could not keep up with my mind, and they would often come out jumbled,” he wrote. “My neck and face would quickly begin to flush a bright red, the searing heat rising all the way to the top of my head; every eye in the room on me; the intense and painful humiliation, and bullying that would follow, all because of my inability to get the words out.”

“The fact that I once stuttered did not keep me from being a successful U.S. Air Force fighter pilot, an airline pilot, or even a public speaker,” Sullenberger added.

Seeing Trump appearing to mock Biden for his stutter caused those memories to rush back to him, Sullenberger wrote, and how that could be detrimental to the self-esteem of children who stutter.

“What might a child who stutters, as I did, feel when they hear a grown-up on a public stage trying to make a bunch of other adults laugh by ridiculing a public figure who also stutters?” he asked.

He concluded the piece by encouraging children who stutter to pursue any goals or career they want.

“You can do any job you dream of when you grow up,” he wrote. “You can be a pilot who lands your plane on a river and helps save lives, or a president who treats people with respect, rather than making fun of them.”

Biden himself responded to Sullenberger’s piece on social media on Sunday morning.

“Thank you @Captsully for sharing your story,” the presidential candidate wrote. “There’s a lot of kids who I bet needed to hear it. Being different isn’t a barrier to success. It can give you the strength to save lives in a crash landing — or even run for President.”

Lara Trump, for her part, denied that she had made fun of Biden’s stutter, but rather chalked up that claim to “egregious reporting we see every day in the mainstream media.”

“Anyone who takes 10 seconds to watch what I actually said can clearly see that I never mention a stutter — didn’t even know he had one — but they can’t help themselves,” she wrote in a tweet Friday evening.