WASHINGTON — As leaders of both political parties and foreign dignitaries publicly mourned John McCain on Sunday, President Trump conspicuously avoided a national moment of tribute to a senator whose death seemed to be its own metaphor for the demise of civility and unity in the Trump era.

The president did not make even the most cursory public show of respect on Sunday for Mr. McCain, against whom he had continued to indulge a personal grievance even as it was apparent that the Arizona Republican was losing his battle with brain cancer. The president spent much of the day golfing and attacking his usual enemies on Twitter.

It was the start of what promises to be a difficult week for Mr. Trump. Mr. McCain quietly declared before his death that he did not want Mr. Trump to take part in his funeral, a decision that will render the president a virtual pariah as the senator is eulogized by former presidents and other luminaries as a principled war hero and dedicated public servant.

But more than just the culmination of a political feud, the specter of Mr. Trump's highly visible absence from Mr. McCain's funeral on Saturday morning at Washington National Cathedral underscored the degree to which the president has veered from the norms of his office, unwilling to act as a unifying force at major moments in the life of the country.

"Everyone, including him, is more comfortable with him not there, and that's a striking thing on its own, given that he is the president of the United States, and this was a sitting senator who is respected by both sides," said Bill Kristol, the conservative commentator and editor at large of The Weekly Standard. "For better or worse, he's outside what would have been the bipartisan boundaries, you might say, of American presidents."

The dynamic reflects a president who wants nothing to do with the establishment and views almost everything as a zero-sum game that revolves around himself.

It also highlights the country's rabid political polarization, which helped propel Mr. Trump to the White House. On Sunday, an admiring tribute to Mr. McCain tweeted by Alexandria Ocasio-Cortez, a progressive Democratic candidate for a New York congressional seat, was greeted by hundreds of vitriolic replies attacking the dead senator and branding Ms. Ocasio-Cortez a sellout and a panderer for praising him.

Some of Mr. Trump's supporters, for their part, savaged Mr. McCain on social media, calling him a spiteful person who had betrayed his own party and blackballed the president as his dying wish. Mr. McCain — whom Mr. Trump once mocked for his five and a half years as a prisoner of war — spent the final months of his life as an outspoken Republican voice challenging Mr. Trump at a time when many in his party would not.

"For most of American history, politics stopped when you had the death of a national leader, and the fact that it hasn't says an awful lot about the current state of our country and our politics, and in particular about Donald Trump," said Michael Beschloss, the presidential historian. "What you'd want to see is a president acting as graciously and as large-mindedly as possible, in the John McCain spirit, but there is no sign of that yet."

Mr. McCain had made his wishes clear during the months before his death, as he convalesced at his ranch near Sedona, Ariz., receiving visitors and fielding telephone calls from a cast of prominent well-wishers across the political spectrum and around the world.

The president was never one of them. His references to Mr. McCain in recent months were confined to contempt-filled moments at his political rallies when he would mimic the thumbs-down signal the senator had made when he voted against repealing the Affordable Care Act.

So intense was Mr. Trump's animus for Mr. McCain that, when he traveled to Fort Drum, N.Y., this month to sign a defense bill named in the senator's honor, the president refused to utter his name. Nor did Mr. Trump join leaders from both parties on Friday in sending sympathy to Mr. McCain and his family after it was announced that he was stopping treatment for his cancer. He died a day later.

On Sunday, flags at the White House were lowered to half-staff to honor the senator, and Vice President Mike Pence wrote on Twitter that "we honor his lifetime of service to this nation in our military and in public life." But Mr. Trump issued no official statement hailing Mr. McCain. He conveyed his condolences to Mr. McCain's loved ones on Twitter on Saturday night, but said nothing about Mr. McCain.

"My deepest sympathies and respect go out to the family of Senator John McCain," Mr. Trump wrote. "Our hearts and prayers are with you!"

In the past, funerals for prominent American political leaders have often served a healing function, Mr. Beschloss noted. John F. Kennedy's funeral in 1963 offered a backdrop for Harry S. Truman and Dwight D. Eisenhower to resolve a more than decade-long feud, as they stood together outside the Cathedral of St. Matthew the Apostle and later sat down for a drink together near the White House.

Before Richard M. Nixon's funeral in 1994, he and his vice president Spiro T. Agnew had not spoken to each other in more than 20 years, as Mr. Agnew held a grudge against Mr. Nixon for pushing him to resign. However, when he was invited to attend, Mr. Agnew agreed.

But when Barbara Bush died this year, Mr. Trump stayed away from a funeral that drew four of the five living former presidents and Melania Trump, who posed in an iconic photograph with her husband's predecessors that seemed to highlight his exclusion from the nation's most exclusive club.

Such periodic comings-together to mourn or recognize a major moment in the nation's history have been a staple of presidential leadership, said Jon Meacham, the presidential historian.

"From Washington all the way through to President Obama, presidents have had to play a unifying and even transcendent role in affirming a sense of national unity," Mr. Meacham said. "It has been and it continues to be almost unthinkable that the 45th president could follow in that tradition, and this is yet another example of his inability to bring disparate forces together even on ceremonial occasions."

Mr. McCain's plan for his funeral — that he be eulogized by both George W. Bush and Barack Obama, the two presidents from opposing political parties who vanquished him in his runs for the White House — has only underscored the contrast, Mr. Meacham added.

"John McCain, in death," he said, "is performing the unifying function that the incumbent president is congenitally incapable of performing."

New York Times, August 26, 2018

Post Script. As the New York Times further reports today, “Senator John McCain, who died on Saturday at age 81, will lie in state in the Capitol Rotunda and receive a full dress funeral service at the Washington National Cathedral." He is one of thirty Americans to be so honored.

As the article above makes clear, there is a growing awareness, in America and across the globe, that the occupant of the White House is morally and temperamentally unfit to lead our nation -in mourning as in living

As Trump once again fails America, it is not surprising that Hillary, who has not stepped loudly into national conversations, is asked by media to become part of the healing.

She recalled and praised Senator McCain, with whom she had served in the Senate.

Here she is on Twitter.

Here she speaks of her friend and colleague by telephone on Meet the Press yesterday.

Here, in person on CNN.

Leaders of the America we love. #TrumpMustGo #HailJohnMcCain

One More Note, if Trump vitriole frequently throws an ugly shadow across America, and you need to remember what America at its best can be, watch John McCain answer one of his supporters during the 2008 campaign, when she declared Obama an “Arab."