The Ruderman Family Foundation, a leading U.S. disability organization, on Thursday strongly denounced Republican presidential candidate Donald Trump for mocking New York Times reporter Serge Kovaleski during a rally on Tuesday.

In videos emerging online, Trump is seen imitating Kovaleski’s disability, who suffers from arthrogryposis, a congenital condition that affects joint movement, during a campaign rally in South Carolina on Tuesday night as he addressed his controversial comments about Muslims celebrating the 9/11 attacks in New Jersey.

“Now, the poor guy — you’ve got to see this guy, ‘Ah, I don’t know what I said! I don’t remember!’ ” Trump said as imitated Kovaleski, whom he knows personally, jerking his arms in front of his body.

“It is unacceptable for a child to mock another child’s disability on the playground, never mind a presidential candidate mocking someone’s disability as part of a national political discourse,” Jay Ruderman, President of the Ruderman Family Foundation, said in a statement. “We also call on him to offer an apology to both Serge Kovaleski and the American public as a whole.”

Calling Trump an individual who’s “so misguided and naïve in his actions,” Ruderman stated, “Our presidential candidates should be moral examples for all Americans and not disparage people with disabilities who make up twenty percent of the American population.”

He also extended an offer to host Trump for “a series of sensitivity training sessions to help him better understand and increase awareness on advocacy towards people with disabilities.”