“I see it in parallel with the way my grandfather’s decline was,” Fred Trump says of Donald’s recent behavior. “If anyone wants to believe that dementia did not run in the Trump family, it’s just not true”

Donald Trump‘s nephew Fred C. Trump III is opening up about his family’s often-overlooked history of dementia — and how his uncle’s recent behavior reminds him of the other Trumps who have faced cognitive health challenges, including Donald’s dad.

“Like anyone else, I’ve seen his decline. But I see it in parallel with the way my grandfather’s decline was,” Fred, 61, tells PEOPLE of Donald, 78, who has at times downplayed his father’s symptoms. “If anyone wants to believe that dementia did not run in the Trump family, it’s just not true.”

As Election Day looms closer, Fred says it’s “frustrating” how the speculation around candidates’ mental aptitude this election cycle hasn’t involved taking a closer look at their family histories. “People (who don’t know the candidates) would diagnose, if you will, Biden, and diagnose Donald, I never heard anyone say, ‘Oh, let’s look at the family history of either of these two people.’ “

The former president’s campaign dismissed the following interview as “completely fabricated and total fake news.”

Fred’s first brush with a loved one showing signs of dementia came when his namesake grandfather, Frederick Christ Trump Sr.was diagnosed with Alzheimer’s disease. He says that Donald’s cousin, John Walter, also had dementia, and Donald’s late sister, Maryanne Trump Barrydisplayed similar symptoms before every death last yeardespite never being diagnosed.

Fred recalls his grandfather’s decline beginning in the 1980s, “with some of the similar stories you’ve heard time and time again, that when his limousine would stop, he would just get out of the car and start walking away.”

Fred Sr.’s behavior became a norm for those around him, says his grandson. “You know, things that you almost say, ‘Oh, there goes grandpa again.’ “

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“You have to understand,” adds the younger Fred, “he still went to work every day — that was part of the thing.”

Fred Sr.’s continued employment “was to give my grandmother (Mary Anne MacLeod Trump) a rest and not have him around the house,” he tells PEOPLE. “My grandmother used to call it the 36-hour day, being around my grandfather, who had onset Alzheimer’s.”

Fred — who described his experience growing up in the Trump dynasty in his recent book, All in the Family: The Trumps and How We Got This Way — recalls his grandfather “becoming a little more agitated” as his condition grew worse.

“He would scream, and I witnessed this, scream at my grandmother for spending too much money,” says Fred, reminding that money was not a concern for Fred Sr. The behavior “kept escalating and escalating, to the point where doctors would say to my grandmother, ‘You really shouldn’t be staying in the same bedroom with him at night.” “

Fred, the son of Donald’s late brother Fred Trump Jr., clarifies that he never saw or heard his grandfather ever become physically violent with his grandmother, but instead, “agitated in his tongue, and vicious.”

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He also alleges that Donald’s sister, Maryanne, seemed to exhibit signs of dementia before her death on Nov. 13, 2023.

“One of the last times I saw Maryanne, she said, ‘You know, I’ve only met your wife Lisa once.’ And I was sort of taken back by that…I said, ‘Well, you’ve actually met Lisa hundreds of times over the almost 40 years that you’ve known her, as long as I have.’ “

Recalling his grandfather’s experience with dementia specifically, Fred claims that he’s witnessed similarities in recent years with his uncle Donald, who has not released any recent medical records as the oldest presidential candidate in US history.

One particular interaction jumps to the front of Fred’s mind as an example, when he ran into Donald at Mar-a-Lago in 2023.

“He just looked disoriented, and he kept repeating something to me time and time again when I had met him,” Fred tells PEOPLE. “He kept repeating things, and he just looked different. He looked tired.”

Fred says he’s noticed his uncle has also had “no inhibitions about cursing in front of anybody” more recently, and while he recognizes that “Donald has always cursed,” his nephew describes the former president’s recent communication as “outright nastiness.”

Related: Fred Trump III Says Kamala Harris’ Rise Is Driving Uncle Donald ‘Absolutely Insane’: ‘It’s Going to Get Nasty’ (Exclusive)

Fred, who decided to back Kamala Harris‘ presidential campaign, explains, “I always have to disclaim, I’m not a neurologist, I’m not a psychologist, I’m not an MD.” But based on his family experiences, he says, “I see traits with Donald that are unfortunate.”

Recalling one of Donald’s first times meeting Fred’s son William, who has complications from a rare genetic mutationFred says Donald rebuffed Lisa’s explanation of William’s condition.

“When Lisa and I went to see Donald, and he said, ‘Well, what’s wrong with William?’ And Lisa says, ‘Well, he has an issue with one of his genes, a gene mutation,’ And Donald immediately says, ‘Not our genes.’ “Fred claims. “He can never admit that there is any mental or physical flaw with anybody in the family.”

Related: Donald Trump’s Nephew Fred Endorses Kamala Harris for President After Making Bombshell Allegations Against Uncle

Donald’s nephew — who claims in his book that the former president once told him disabled Americans “should just die” due to “expenses” and “the shape they’re in,” which Donald’s campaign is firmly denied — tells PEOPLE he thinks the former president is likely playing up “the old projecting thing” by denying any flaws in his family’s medical history.

“He can never open up the negative door on the family ever,” says Fred. “Because if he admitted that my grandfather had dementia, well, that’s almost admitting that he has the possibility of having dementia.”

In a statement responding to Fred’s criticisms, Donald told PEOPLE that he has offered his nephew a lot of support, “more than anyone else in his life, and this is the thanks I get.” Donald’s son Eric added in a statement that his cousin’s claims are “garbage.”

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The potential that Americans could elect Donald to the White House once again without certainty about his cognitive health has Fred concerned, following a series of public campaign trail flubs in which the former president has mixed up names of world leaders, slurred his speech and relied on increasingly violent rhetoric about his opponents.

“My grandfather, unfortunately, was threatening my grandmother (late in his life), and that was horrible. And I witnessed it,” Fred says. “But my grandfather didn’t have any decision-making that was going to have world implications. The biggest implication is, ‘Does my grandmother sleep in another bedroom to stay away from him when he’s roaming around the house at two o’clock in ‘the morning?’ “

But for Donald, he adds, “this is potentially the most important person in the world.”