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

Fred C. Trump III believes that the worst is yet to come this election season as his uncle Donald Trump looks for ways to save his presidential campaign.

"I don't pretend to be an armchair psychologist, I just know him," Fred, 61, tells PEOPLE. "It's 'win at all costs' and he doesn't care who gets hurt in the wake of what he's trying to get accomplished, which is to benefit himself."

Fred, who recently endorsed Kamala Harris for president, says that Joe Biden's late departure from the race seems to have caught Donald's campaign "flat-footed."

"How they didn't anticipate that Harris could become the nominee?" he says. "I just don't understand how they didn't have something, a plan B, ready."

Since Harris took Biden's place at the top of her party's ticket, Democrats have seen a dramatic surge in enthusiasm. Donald — who was the clear frontrunner when matched up against Biden — is now trailing Harris in most national polls, and he's lost his lead in multiple must-win swing states as he struggles to recalibrate his campaign.

"He's looking very tired," Fred observes.

Asked how he imagines Donald, 78, is reacting as Harris' rise dominates headlines, Fred says, "It's driving him nuts. ... It's driving him absolutely insane."

"I wouldn't want to be around him right now because it's never his fault," he continues. "All bets are off now, he is going to go nuts on people."

Among those who will be targeted by the former president, Fred predicts, are Harris and anyone who sides with her over Donald.

"It's going to get nasty. The race is going to come up in a big ugly way," he says. "[Donald] demeans people on a regular basis. ... Everybody's a loser. Everybody is stupider than him. You've seen it already."

Fred made waves in July with his new book, All in the Family: The Trumps and How We Got This Way, which features stories about his famous family, including the good and bad times with Uncle Donald.

"He's the one who introduced me to playing golf. I knew him through his school years, college, his business career. I actually had my first two summer jobs working for the Trump organization," Fred tells PEOPLE. "I'm not going to say that it was all horrible with Donald, it wasn't — but he's done horrific things to me."

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In one shocking part of the book, which Donald's spokesperson dismissed as "completely fabricated," Fred claims that his uncle once said in the Oval Office that severely disabled people "should just die" because of "the shape they're in" and "all the expenses."

The comment allegedly came after Fred — whose son, William, has complex intellectual and developmental disabilities — brought experts to the Trump White House to share how the government can better support disabled people and their families.

On a separate occasion after that, Fred alleges in the book, Donald made a more pointed statement when his nephew asked for help covering the costs of William's care. According to Fred, Trump replied: “I don’t know. He doesn’t recognize you. Maybe you should just let him die and move down to Florida.”

Talking to PEOPLE about Donald's alleged remarks, Fred says that William does not speak or walk, and he requires a lot of assistance. "But there's somebody within him," he adds. "There is."

Fred believes that a Harris administration would take his disability advocacy more seriously, and he plans to "hold their feet to the fire" if she's elected until progress is made.

As for now, Fred is ready to assist Harris' campaign where he's needed if it helps prevent a second Trump administration. Writing All in the Family was one way that he felt he could shed light on the concerns he has about his uncle.

"I see what I truly think he's planning to do to this country, should he win or if he loses, so I needed to voice it from somebody who knows him personally," Fred says, noting that he's worried about either outcome of the election after watching the "grotesque" Jan. 6, 2021, insurrection at the U.S. Capitol when Donald lost in 2020.

He continues: "We need to get back to civility and to a future that is not dystopian in the way I foresee it becoming if Donald is elected."