US President Joe Biden will use a visit to Hanoi to boost his image as a unifier by saluting the memory of John McCain -- Vietnam War hero, Republican stalwart, and serial antagonist of Donald Trump.

Biden begins a two-day visit to Vietnam on Sunday to deepen diplomatic and economic ties with the communist state in the face of growing Chinese assertiveness and concerns about supply chain vulnerabilities.

But with the 2024 election rapidly approaching, Biden is keeping one eye on the campaign even while abroad.

The 80-year-old, gunning for a second term, will Monday visit the memorial to McCain marking where his plane was shot down on October 26, 1967.

Seriously wounded, fighter pilot McCain -- the son and grandson of admirals -- was taken prisoner of war, incarcerated for more than five years and tortured.

After the war he became involved in politics for the Republican party, serving as a US senator for several decades and running for president in 2008, when he was defeated by Barack Obama -- and his running mate Biden. 

McCain also earned great respect from people in Vietnam for his work in building bridges between the former foes.

- Friendship - 

It was on the Senate benches that McCain, known for his independent spirit, became friends with Biden, a Democrat with a very different background from his own.

Biden did not fight in Vietnam, having been exempted from the draft because of his studies and asthma -- but he also did not protest against the war as so many young Americans of his generation did.

Already a centrist at heart, and respectful of institutions, the student Biden was the type to wear a jacket rather than the tie-dye T-shirts emblematic of the hippie movement, he told reporters in 1987.

"Other people marched. I ran for office," he added, according to The Washington Post. In 1972 he was elected to the Senate at the age of 30.

By paying tribute to McCain, the president is marking a difference with the man who could well be his opponent again next year: his predecessor Trump. 

When McCain died of brain cancer in 2018, Biden delivered an emotional eulogy at his funeral, stressing that their friendship transcended political differences. 

"My name is Joe Biden. I'm a Democrat. And I loved John McCain," he said, praising McCain's "ageless" code of honour.

He hailed him as an example of "the American story, grounded in respect and decency, basic fairness, the intolerance of the abuse of power".

- Trump on the golf course - 

Biden had already privately decided to run against Trump and his eulogy denounced -- if not explicitly -- the mercurial billionaire, whom McCain despised.

Trump -- who was similarly open in his dislike of McCain -- played golf on the day of his funeral.

Later McCain's widow urged voters to back Biden, further fuelling Trump's resentment.

"I never was a fan of John McCain, and I never will be," Trump said in March 2019.

In paying tribute to the former war hero on September 11 -- a date etched in the American consciousness because of the terror attacks of 2001 -- Biden will also be looking to burnish his patriotic credentials.

After leaving Vietnam, the president will go to a military base in Alaska to commemorate the attacks. An unusual choice of location, if not completely unprecedented -- more usually the "Commander in Chief" visits New York or one of the other sites hit on 9/11.

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