late US senator John McCain will receive his final public sendoff Saturday in a nationally televised ceremony featuring eulogies from two ex-presidents, but with current commander in chief Donald Trump conspicuously absent from the proceedings.

Republican George W Bush and Democrat Barack Obama will deliver remarks honouring their friend and former White House challenger, at a memorial service in Washington's National Cathedral that McCain planned himself in recent months as he battled brain cancer.

That the men who vanquished McCain in their presidential battles were asked to speak is testament to the former war prisoner's commitment to looking beyond party and signalling that Americans, regardless of political affiliation, are rowing together in the same boat.

Amid today's inflammatory political environment the message could serve as a soothing balm for a nation bruised by two years of divisive discourse.

And the absence of Trump, whose bitter feud with McCain has wrangled US politics during that time, will serve as a final rebuke of the president, highlighting the clash between a Republican elder statesman and the current president from his own party.

McCain's last public event, before he is laid to rest Sunday in a private ceremony at the US Naval Academy in Annapolis, Maryland, comes a day after he was accorded the rare recognition of lying in state in the US Capitol.

While members of Congress honoured one of their own in a touching ceremony that featured an address by Vice President Mike Pence, Trump was again not there.

Instead he holed up in the White House and remained uncharacteristically silent on Twitter during the ceremony, before flying to a political event in North Carolina later in the day.

McCain's widow Cindy, his seven children and his 106-year-old mother Roberta McCain joined scores of members of Congress, state governors, diplomats and other dignitaries at the somber Rotunda ceremony.

Pence, in his tribute, told McCain's family that "it is deeply humbling to stand before you today at the United States Capitol to commemorate the life and service of an American patriot."

"The president asked me to be here, on behalf of a grateful nation, to pay a debt of honor and respect to a man who served our country throughout his life, in uniform and in public office." It was an awkward message to deliver from a president who has studiously refrained from praising McCain, either during his illness or since his death.

Their feud took root during Trump's 2016 campaign, when he questioned the notion McCain was a war hero -- because he had been captured after his navy fighter jet was shot down over Hanoi in 1967.

McCain pushed back in the following months, calling Trump's behaviour petty and "disgraceful," and in one of his final acts in the Senate blocked the Republican effort to repeal Obama's health care law known as Obamacare.

McCain the aviator spent more than five years in a Vietnamese prison camp, returning home to launch a political career that saw him eventually run for president in 2000 but lose the nomination to Bush.

Eight years later, he won the nomination in a contest that seemed almost predestined -- only to lose the election to Obama, who became America's first black president.

Saturday's ceremony could serve as a rehabilitation of sorts for Bush, who will be delivering one of his most high-profile addresses since leaving the White House nearly 10 years ago.

He has endured deep criticism for controversially leading the US into war in Iraq -- an invasion that McCain steadfastly supported at first, but eventually grew to believe was a mistake.

For Obama, the moment will allow him to share his thoughts about a presidential campaign rival whose magnanimity in defeat only boosted his stature as an American statesman.

"Tonight more than any night, I hold in my heart nothing but love for this country and for all its citizens, whether they supported me or Senator Obama," McCain said at the time.

"I wish Godspeed to the man who was my former opponent and will be my president.