OPINION — The adjective “graceless” has been part of the language for more than six centuries. A word frequently deployed by Shakespeare — “What is she but a foul contending rebel, And graceless traitor to her loving lord?” — it has never had a more appropriate use than in describing Donald Trump’s graceless lack of respect for John McCain.

Nothing better symbolizes the cruel vulgarity of the Trump administration than the president’s refusal to offer a public comment for 48 hours on McCain’s death beyond a barebones tweet combined with the White House raising the flag to full staff after midnight on Monday.

Back in the “Mad Men” era, it was an advertising cliché to say, “Let’s run it up the flagpole and see who salutes.” Trump instead ran it up the flagpole to see who heckled. Only when veterans groups like the American Legion objected did Trump give McCain the honor due a war hero and revered senator.

Watch: McCain vs. Trump — Can the President Give Up the Spotlight?

Teflon Trump

All of this points to what, for me, is the central mystery of the Trump presidency: Why do his supporters put up with it?

Any generic Republican president (say, Jeb Bush or Marco Rubio) would have slashed taxes, rolled back regulations and appointed conservative judges. But no honorable man or woman in public life would have … hang on, it’s a long list:

Ridiculed a Vietnam POW. Lied compulsively about everything. Trusted felons like Paul Manafort, Michael Flynn and Michael Cohen. Viciously and repeatedly attacked the integrity of the FBI, the Justice Department and the national security establishment.

Declared war on a free press. Cozied up to authoritarian leaders like Vladimir Putin. Kept secret what happened at the Helsinki summit. And continually treated Canada (yes, Canada) like a major enemy.

The list is far from comprehensive and skips pre-presidential actions like approving hush money for a porn star. But, to steal a line from Bob Dole’s 1996 presidential campaign, “Where’s the outrage?”

What is it about Trump that has the power to cloud the minds of Republicans who should know better?

Is it the president’s hatred for any immigrant who does not hail from Norway (with the exception of a certain former Slovenian model)? Is it his rabble-rousing vitriol directed at black athletes, especially at NFL players who kneel to protest police brutality?

Economic uncertainty in the Rust Belt can only go so far in explaining Trump’s Icarus-like poll numbers among GOP voters. For all the White House hype, the president’s trade wars have cost more American factory jobs than they have created. And don’t get me started on that geo-political genius in the White House who was snookered by empty promises on nukes from the Rocket Man in North Korea.

Belatedly watching the Netflix series “The Crown,” I was struck by how faithfully young Queen Elizabeth II adhered to Britain’s unwritten constitution that circumscribes the power of the monarch. Before Trump, America too had an unwritten constitution of norms that no president would even think of defying.

In prior administrations, it never occurred to anyone to yank the security clearance of a former official who criticized the president on television. Or that a president could get away with profiting from a hotel near the White House curiously popular with foreign governments. Not to mention Trump’s unprecedented refusal to release his tax returns.

Trump defenders go to extremes of double-jointed moral flexibility to defend his contempt for democratic traditions. In a dispiriting New York Times op-ed, Charles R. Kesler, a senior fellow at the conservative Claremont Institute, argued, “Most of Mr. Trump’s alleged transgressions … offend against the etiquette of modern liberalism and modern liberal governance, not the Constitution.”

Forgive me, I thought that human kindness and a sense of shame were conservative values.

Recalling Vietnam

The 50th anniversary of the tear-gassed 1968 Democratic Convention in Chicago offers a reminder that America has survived turbulent eras before. The passions unleashed by the tragic, misguided war in Vietnam that left more than 58,000 names on a wall in Washington were re-enacted in every presidential campaign until McCain won the Republican nomination in 2008.

For many male journalists of my generation (particularly those of us who were former Vietnam protesters), an undisguised admiration for McCain was partly a way to atone for the antiwar fanaticism of the era.

For all the cynicism of the Johnson and Nixon presidencies in their refusal to admit the war was lost, too often students on the barricades at home refused to acknowledge — let alone respect — the heroism and love of country of those who served in America’s saddest war.

Just like McCain admitted when he was wrong (apologizing for backing the Confederate battle flag in the 2000 South Carolina primary) so did reporters like myself admit that we had been wrong about those who proudly served America in uniform during the 1960s.

Maybe that’s why Trump’s petulant scorn for McCain so rankles. Here is a president, whose “personal Vietnam” was avoiding sexual diseases at Studio 54 in the 1970s, refusing to honor a senator whose suffering during more than five years in captivity was beyond the imaginings of almost all of us.

One of the most moving tributes to McCain came from John Kerry, a Vietnam veteran (and passionate war critic) who worked with the Arizona senator to restore normal diplomatic relations with Vietnam. As Kerry recalled, “We traveled together to Vietnam and together, we found common ground in the most improbable place.”

Common ground. That’s the part of American democracy that Trump will never understand. That sometimes you join hands with your former adversaries in the quest to make this a country — to steal a McCain book title — “worth the fighting for.”

Walter Shapiro, a Roll Call columnist since 2015, has covered the last 10 presidential campaigns. He is also a fellow at the Brennan Center for Justice at NYU and a lecturer in political science at Yale. Follow him on Twitter @MrWalterShapiro.