WASHINGTON - Donald Trump’s conspicuous failure to pay tribute to the late John McCain Monday underscored the isolation of the US leader - seen by critics as incapable of bringing a divided nation together even as it mourns a political icon.

READ MORE: Going not-so green

The US flag at the White House was back at full staff Monday morning - in contrast to the US Capitol - after being lowered over the weekend to mark McCain’s death Saturday of brain cancer at age 81.

A war hero who endured torture as a prisoner of war in Vietnam, the Arizona Republican was an uncompromising voice on issues of war and peace in the US Congress and a two-time candidate for the presidency.

But Trump, who bypassed military service himself, has had only words of contempt for the senator, famously dismissing him during the 2016 presidential campaign as “not a war hero.”

“He’s a war hero because he was captured. I like people that weren’t captured,” Trump said.

The bad blood was mutual and on display even in death. Battling cancer, McCain reportedly excluded Trump from his funeral arrangements.

Trump won’t attend funeral

US President Donald Trump will not attend funeral ceremonies for John McCain in Washington this weekend, a spokesman for the late senator confirmed on Monday.

“The president will not be, as far as we know, attending the funeral. That’s just a fact,” longtime aide Rick Davis told a press briefing in Arizona.

Trump’s only comment so far has been a single, terse tweet, even as the US Congress prepares to give McCain a statesman’s send-off.

Trump sent his “deepest sympathies and respect” to the bereaved family Saturday, but offered no words for McCain himself.

No White House statement

The Washington Post reported that White House Press Secretary Sarah Sanders, Chief of Staff John Kelly and other senior staff had urged a statement be released referring to McCain as a “hero.”

The statement was given to Trump for his approval, but according to the paper, which cited current and former White House aides, the president said he preferred to tweet instead.

The absence of a formal White House statement has underscored Trump’s isolation in a city where McCain’s death otherwise has been a rare occasion for bipartisan praise for his lifetime in public service.

He will lie in state at Arizona’s capitol on Wednesday, before a public viewing Friday at the Rotunda of the US Capitol, an honor reserved for the likes of John F. Kennedy, Ronald Reagan and civil rights champion Rosa Parks.

A national funeral service will be held Saturday at the National Cathedral in Washington, with lawmakers, foreign leaders and other dignitaries in attendance. Former presidents George W. Bush and Barack Obama - a Republican and a Democrat - have been asked to deliver eulogies. The current president’s likely absence could not be more conspicuous.

Tributes have poured in from every living former president, honoring the former Navy aviator for his courage, integrity and decency.

Vice President Mike Pence and First Lady Melania Trump offered their condolences and thanks “for your service to the nation.”

Obama hailed his former rival for the US presidency in 2008 as a man of courage who showed what it meant “to put the greater good above our own.” The New York Stock Exchange observed a moment of silence in McCain’s memory.

At bitter odds

From the day the billionaire former reality TV star announced his candidacy for the Republican nomination with an attack on Mexican immigrants as criminals and rapists, Trump and McCain have been at bitter odds.

McCain denounced Trump for using language that “fired up the crazies,” while Trump said McCain was a “dummy” who barely managed to graduate from the US Naval Academy.

In the years since, McCain has been Trump’s loudest - and often a lonely - Republican critic, especially as the president disrupted America’s longtime alliances.

In McCain’s eyes, Trump’s meeting in Helsinki with Russia’s President Vladimir Putin earlier this year was “one of the most disgraceful performances by an American president in memory.”

McCain meanwhile won the president’s undying enmity last year, by casting a dramatic thumbs down vote - only one of three Republican senators to do so - against a Trump-backed effort to repeal Barack Obama’s signature health care law.

Trump, who regularly denounces that vote at his rallies, earlier this month repaid his nemesis by signing the John McCain National Defense Authorization Act without ever speaking the name of the person it was meant to honor.

Posthumous message

The late US senator John McCain warned against “tribal rivalries” that have sown “hatred” around the world, in a final message read posthumously Monday that appeared aimed at President Donald Trump.

“We weaken our greatness when we confuse our patriotism with tribal rivalries that have sown resentment and hatred and violence in all the corners of the globe,” McCain said in a poignant farewell statement read by his campaign manager Rick Davis.

“We weaken it when we hide behind walls, rather than tear them down, when we doubt the power of our ideals, rather than trust them to be the great force for change they have always been,” he added in an apparent reference to the Trump presidency.