(CNN) Radio shock jock Howard Stern believes President Donald Trump's candidacy for president started out as a "publicity stunt" in order to get a raise from NBC for hosting "The Apprentice."

Stern, who has hosted Trump on his radio show for years, told CNN's Anderson Cooper in an interview set to air in full on Friday that he has "no doubt" it was a publicity stunt. Stern told Cooper that he has "inside information" that Trump started out using rumors that he would run for president as a public relations tactic when he was promoting his 1987 book, "The Art of the Deal."

He said at first, it was a "PR guy's idea. He said, 'Donald, what you need to do is we'll ... make sort of a rumor that you're running for president.'"

"So all of a sudden he was being interviewed," Stern said. "The book goes right to No. 1."

Stern said when Trump's second book came out, "that's when he decided to start the rumor that he was gonna run for president."

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