In fact, Mr. Trump's predecessors, going back to George Washington, have all tried, with varying degrees of success, to summon Americans to a higher moral purpose. [...]

If anything, the president's role as a moral arbiter has only deepened in recent decades, as American society has become more secular, more racially and ethnically diverse, and more atomized by the splintering of mass media and the rise of social media.

"Especially in modern times, with instant communications, many Americans are inclined to look to a president for guidance on what to think about crucial issues," the historian Michael Beschloss said. "A president must always speak with an intense degree of moral sensitivity."

When Jimmy Carter was asked in 1978 whether he planned to use the moral weight of his office to block American neo-Nazis from marching in Skokie, Ill., a predominantly Jewish suburb of Chicago, he replied that it was up to the courts. But he added, "I wish that this demonstration of an abhorrent political and social philosophy would not be present at all."

Ronald Reagan famously spoke of America as a moral lodestar -- a "shining city on a hill." In his farewell address in 1989, he said, "She's still a beacon, still a magnet for all who must have freedom, for all the pilgrims from all the lost places who are hurtling through the darkness, toward home."

George W. Bush, addressing Congress days after the terrorist attacks of Sept. 11, 2001, issued a call to arms against the Islamic extremists who brought down the World Trade Center. But he coupled it with an appeal to avoid a broader attack on Islam.

"We respect your faith," Mr. Bush said. "It's practiced freely by many millions of Americans and by millions more in countries that America counts as friends. Its teachings are good and peaceful, and those who commit evil in the name of Allah blaspheme the name of Allah."

Mr. Trump's predecessor, Barack Obama, appealed to the best in Americans through a heartbreaking succession of police shootings and racially motivated killings. He often invoked the notion of grace -- never more indelibly than in Charleston, S.C., after a white supremacist gunned down nine people, all African Americans, during a prayer service at Emanuel African Methodist Episcopal Church.

"We don't earn grace, but we choose how to receive it. We decide how to honor it," Mr. Obama said, before leading a memorial service in the hymn "Amazing Grace."