In an interview airing Sunday at 8 p.m. ET on "Life, Liberty & Levin," the president's eldest son promoted his new book, "Triggered," and claimed Americans are fed up with the repeated investigations into his father.
"They're trying to, you know, reverse the results of an election that they didn't agree with, though the American people obviously agreed," he said, at one point characterizing the past several months as a "witch hunt."
Trump recalled the Russia investigation, which lasted more than two years and culminated in a marathon Capitol Hill hearing featuring Mueller, the former FBI director under Presidents George W. Bush and Barack Obama, and a decorated veteran of the Vietnam War.
"Obviously, [Democrats] only brought him in because he was the former head of the FBI and he was a decorated Marine," Trump Jr. said. "So he's above reproach. Trump cannot possibly attack him."
"Then, they were going to get him, finally -- they were going to get Trump. We're gonna put him on the stand. And, within about 30 seconds, the American public realized that this thing was one big joke, that they used Robert Mueller simply because of the credentials and ... the fact that he was beyond reproach because of his resume. They realized this guy had no idea what he's even talking about."
Trump Jr. claimed Mueller's testimony showed he had inadequate knowledge of the contents of the report that bore his name.
"It was a joke," he said. "And I do believe that despite the media attacks, the American people get it ... They see the nonsense. They see the witch hunt."
In his interview with host Mark Levin, Trump appeared to reference an exchange between Rep. Matt Gaetz, R-Fla., and Mueller during the July hearing in which the former special counsel claimed allegations surrounding ex-MI6 agent Christopher Steele was a subject "beyond my purview."
Responding to Mueller, Gaetz fired back: “No, that’s directly in your purview Mr. Mueller and here’s why. Either Steele made this whole thing up and there were never any Russians telling him of this vast criminal conspiracy that you didn’t find, or, Russians lied to Steele.”
In his opening statement before the House Judiciary Committee, Mueller emphasized that his investigation "found that the Russian government interfered in our election in sweeping and systematic fashion."
"Over the course of my career, I’ve seen a number of challenges to our democracy," Mueller said later. "The Russian government's effort to interfere in our election is among the most serious."
On the whole, Trump told Levin, his father has shown Republicans how to wage political war with Democrats -- whether it be during the Russia investigation or impeachment inquiry.
"And I think it's been a great thing in the sense that it's shown conservatives that you can actually fight back. You don't just have to turn the other cheek," he said. "We've done that for 50 years and all we've done to cede ground to the lunatic fringe of the left and their policies."
On Sunday, "Life, Liberty & Levin" moves to a new time: 8 p.m. ET on Fox News Channel.
Fox News' Brooke Singman and Vandana Rambaran contributed to this report.