And he knows the words to Waltzing Matilda.

Senator McCain, the 81-year-old chairman of the US Senate armed services committee, is being treated for brain cancer while continuing his work in the Congress.

He is the dominant figure in Senate policy on the US military and a political giant in the American Congress. He has long been Australia's key Capitol Hill ally when Canberra needed help. And along the way he adopted an understudy, a fellow Republican.

"Senator McCain encouraged me to take the lead on Asia-Pacific strategy and force posture on the Senate Armed Services Committee," says Dan Sullivan of Alaska.

Elected to the Senate a little under three years ago, he's a full generation younger than his mentor at age 53. Yet he has already established himself as a force: "Dan Sullivan has emerged as the Senate armed services committee's 'go-to' guy on security issues in the Pacific," says Mike Green of the Centre for Strategic and International Studies in Washington, a theatre that includes China, Japan, the Korean Peninsula and Australia.

McCain was an early critic of Donald Trump when he was shaping to run for the Republican nomination for the presidency. After he described the people at a 2015 Trump rally as "crazies", Trump responded by disparaging his war record.

The then navy pilot was shot down, tortured and held captive by the North Vietnamese for six years. "He's a war hero because he was captured?" said Trump. "I like people who weren't captured."

Trump avoided serving in Vietnam by seeking a series of deferments, student and medical.

Sullivan, too, served in the military, but in the marines. Which is how he came to know Waltzing Matilda, adopted as the official march of the US 1st Marine Division when it took leave in Australia during the Pacific War.

Incidentally, Sullivan has been involved in a range of key Australian dealings with the US in recent years in various incarnations in the US government.

The Harvard-trained lawyer helped negotiate the Australia-US Free Trade Agreement as an assistant to US national security adviser Condoleezza Rice.

He helped negotiate the airline access deal, the so-called Open Skies agreement, between the two countries as a diplomat at the State Department.

And he worked with Australian forces in Afghanistan and Pakistan as a staff officer to General John Abizaid of US Central Command in 2004-06.

Green says that he has "particular influence" on US affairs despite his first-term Senate status, a result of his experience in government, his service in the marines, and his affiliation with McCain.

He is consulted by the Trump White House and major regional leaders including Japan's Shinzo Abe and China's Xi Jinping.

And his view of the region?

"We have the immediate challenge with North Korea, but long-term the most important geostrategic issue facing the US and our allies is the rise of China and how to address it," he tells Fairfax Media.

"I know it's a huge issue in Australia too, how to deal with China from an economic and military standpoint. You seem to alternate in emphasis - 'our economic future is with China, our national security history has been with the US, and are these trends in tension?'"

On North Korea, Sullivan successfully negotiated a boost to US spending on missile defence. On China, he advocates more robust patrols of the South China Sea and a much tougher line on economic policy. And he would like Australia to help.

"I'm not saying we should contain China; US strategy has always been to engage China. We need to look at much stronger reciprocity. China owns the Port of Darwin, where US marines are. Could an Australian company go into China and buy a Chinese port? I doubt it.

"If we can't invest there, they shouldn't be able to invest here. The joke's over."

Sullivan said he was making his case to the Trump administration and other senators. "It would be enormously helpful" if allies including Australia took the same approach.

That is a tune that neither Malcolm Turnbull's Coalition nor Bill Shorten's Labor are ready to sing along to.