No president in American history has been in more desperate need of a moderating influence than Donald Trump, whose professional and temperamental deficiencies in the office are laid bare on an almost daily basis. His presidency is threatened by his unwillingness to fully embrace the long-established White House tradition of the presidential right-hand man.

Every president at some point has one. Washington had Hamilton. Bill had Hillary. Lincoln had Seward. Reagan had three at the same time. FDR went through them like Kleenex. He would lift them up plump and pink from where they were in the government at the time and return them afterward, desiccated approximations of their former selves.

White House Chief of Staff John Kelly once seemed destined to be Mr. Trump’s right hand, but the president never truly empowered him, and the general is on increasingly shaky ground after bungling the resignation of White House Secretary Rob Porter amid allegations of domestic violence. Today, Mr. Kelly is a favorite punching bag of Anthony Scaramucci — reportedly with Mr. Trump’s blessing — as the former White House communications director makes the rounds on the talk show circuits.

New to the business of governing and not being a student of American history, Mr. Trump may not understand the historical importance of a right hand to the proper functioning of the office. It’s about more than just keeping the trains running on time. The special class of White House adviser has traditionally functioned as a personal mouthpiece for the president. They have shielded him from blowback when he does something stupid, and they fluff his sagging ego before he steps out into the spotlight. They have been his conscience, his chorus and his confidant.

Through time, the role has evolved, adapting to the shifting authorities and needs of the presidency to become a central element of his power. The catalog of these special partnerships is long and illustrious.

Alongside Washington and Hamilton and Lincoln and Seward, there was the collaboration between President Woodrow Wilson and Col. Edward House that endured a world war and facilitated the creation of the United Nations before petty jealousies and misunderstandings drove them apart.

FDR’s first in a long line of right-hand men and women, the gnome-like and unmistakably working-class Louis Howe, was so strikingly dissimilar from the young, patrician Roosevelt when they first met that many people were surprised they even knew each other. Yet their collaboration became one of the most important in U.S. presidential history as the New Deal programs they created helped lift millions of Americans out of poverty during the Great Depression.

And the uncommon power Sherman Adams enjoyed as President Eisenhower’s chief of staff and right hand stands unequaled to this day. During the short period in which the president recovered from his heart attack, his trusted aide Adams secretly ran the government in his place.

None of our presidents has been perfect, but the best of them have recognized their flaws as leaders and permitted their right-hand men and women to compensate for them in those areas where they were weak.

President Reagan knew managing detail was not his strong suit so he permitted people like his “Troika” of key advisers, James Baker, Michael Deaver and Edwin Meese, to act as another set of eyes for anything of consequence placed before him. They sat in on every significant meeting Reagan attended and they read every policy recommendation or memo he did.

President Eisenhower was a foreign affairs master, but he was bored by domestic issues so he placed his chief of staff at the head of his domestic policymaking architecture.

President Truman was legendarily given to making embarrassing gaffes during his public statements. He often permitted his right-hand man and eloquent White House counsel, Clark Clifford, to act as his personal spokesman when it was appropriate.

And, the discreet and dignified President Washington permitted pugilistic Alexander Hamilton to fight his petty public battles for him with the press.

President Trump has deficiencies in all of these areas. He would do well to consider how his predecessors dealt with their flaws as leaders.

If the Trump Presidency is to be regarded as successful, the president must do what all of the great presidents have done before him and open his eyes to the ways he is threatening his legacy — and listen to his right-hand man, whoever that may be.

K. Ward Cummings (kwardcummings@gmail.com) is the author of “Partner to Power: The Secret World of Presidents and their Most Trusted Advisers.”