It’s existed since Eisenhower, and has been essential from Nixon onwards. Yet, the role of White House chief of staff is being “defined out of existence” by President Trump, according to journalist and author Chris Whipple.

“Donald Trump has seemed to have learned all the wrong lessons from the first two years of his presidency,” Whipple, who wrote The Gatekeepers: How the White House Chiefs of Staff Define Every Presidency, told me for the latest episode of “Between You & Me.” “He still thinks he’s the smartest guy in the room, who knows how to govern, and if everyone would just get out of his way, everything would be fine. The truth is the opposite.”

The high frequency of White House turnover has been well documented, and chiefs of staff haven’t been immune. Trump has already gone through two; after less than seven turbulent months he ousted former RNC chairman Reince Priebus (remember him?), and at the end of last year he ditched former Marine General John F. Kelly after unsuccessfully downplaying months of rumors about tensions between the two men. On his way out of the West Wing, Kelly made a pseudo-defense of his tenure to the LA Times, arguing that he should be judged not by what Trump did but what he didn’t do during that time.

Whipple finds that excuse deeply unconvincing.

“John Kelly had, no question about it, the toughest assignment of any White House chief in modern history. There is nobody like Donald Trump, and there is no bigger challenge,” he told me. “But I think that Kelly made a fundamental mistake and that is he reinforced all of Donald Trump’s worst partisan instincts.”

By Whipple’s count, there have been 18 chiefs of staff in history — 19 if you include Trump’s acting chief Mick Mulvaney, the director of the Office of Management and Budget, who took on the role at the start of the new year. Indeed, Mulvaney was spotted passing around Whipple’s book The Gatekeepers in the Oval Office at the White House last month. Mulvaney reportedly referred to passages in the book about how to effectively manage the West Wing.

For Whipple though, Mulvaney — who has already been defending his boss for “executive time” and for dubiously declaring a national emergency to secure money for a border wall — has not been putting the theory into practice.

“It’s almost as though he read the book and did the opposite of what I’ve advocated,” Whipple lamented. “The most important thing a White House chief of staff can do is to tell the president hard truths, and Mulvaney right from the get-go seems to have abandoned that idea.”