About two-thirds of Australia's merchandise trade and 30 per cent of the $US19 trillion ($27 trillion) in global trade passes the South China Sea.

Speaking on the campaign sidelines to The Australian Financial Review, Mr Rubio's spokesman, Alex Conant​, signalled that, if elected, a president Rubio would look to Australia for support in dealing with China.

"We're going to put our allies first and we're going to invest in our military," he said.

"Especially in Australia, I think you'll see reinvestment in our navy, reinvestment in our relationship with Australia and our other allies in the Pacific, including the Japanese."

"We're not going to let China take over parts of the South China Sea, without any responsibility."

Team Rubio's hawkish critique of China, Australia's largest trading partner, underlines how Canberra is likely to meet a more assertive American defence ally after November's presidential election.

Assertive America

Democratic nominee front runner, Hillary Clinton, though less openly hostile towards Beijing than Senator Rubio, holds more hardline views towards China than Mr Obama, according to former senior government figures and diplomats who have dealt closely with her.


Sections of the Australian business community have urged the government against inflaming tensions with China, arguing it could hurt commercial ties.

Prime Minister Malcolm Turnbull spoke to Senator Rubio and Mrs Clinton over the telephone during his visit to Washington last month, when he also discussed international security and trade issues with President Obama at the White House.

Mr Turnbull and Senator Rubio agreed on the need for continued US engagement in Asia, including through the Trans-Pacific Partnership trade and investment accord.

Senator Rubio in recent years has also met the leaders of Japan, South Korea and Philippines, who have all expressed concern about China's aggressive construction on reefs, drilling for oil and unilateral land reclamations in disputed waters in the South and East China Seas.

In the US election race, Senator Rubio exceeded expectations to finish third in the Republican caucus in Iowa last week. He has crept up in polls to second behind Mr Trump in New Hampshire, which holds its primary vote on Tuesday.

Though Mr Trump leads in national polls, Mr Rubio has been backed into warm favouritism in betting markets to win the Republican nomination.

John Lee, a conservative defence scholar at the Australian National University and the Hudson Institute in Washington, said Senator Rubio's China comments "might be seen as a little hawkish but they accord with both official and public sentiment in Australia" on Beijing's behaviour.

"A more assertive American posture and policy is now seen as part of the solution when it comes to managing and constraining Chinese power, rather than as a destabilising development," Dr Lee said.


Mr Turnbull said in Washington last month that US military and economic engagement in Asia was crucial in underwriting peace and prosperity in the region.

The US currently has about 1200 marines stationed in Darwin, and the number is due to gradually rise to 2500, as part of President Obama's "rebalance" to Asia.

However, there has been niggling disagreement between the US and Australia over splitting the cost of building new facilities for the extra marines, and a separate minor dispute over cost-sharing for a bigger runway and refuelling facilities for US airforce bombers, a security source said.

South China Sea challenges

The Turnbull government has endorsed the Obama administration's recent freedom of navigation exercises and flight challenges by the US navy and airforce within 12 nautical miles of artificial islands claimed by China.

Smaller Asian neighbours, including Vietnam, the Philippines, Malaysia, Taiwan and Brunei dispute China's claims.

Senator Rubio, a member of the influential US Senate Foreign Relations and Intelligence committees, has vocally backed the Obama administration's freedom of navigation exercises and South-east Asia Maritime Security Initiative to build the maritime capabilities of countries in the region.

In November, the US dispatched the USS Lassen, a destroyer, within 12 nautical miles of Subi Reef, an artificial island constructed by China.


The sail by, condemned by Beijing, was designed to show the US does not recognise Chinese claims to the seas.

"That's the sort of thing we need to make it clear we're not going to let China take over the South China Sea without any response," Mr Rubio's spokesman said on Friday. Mr Conant previously worked for president George W. Bush.

The Turnbull government approved a low-key freedom of navigation flight by a Royal Australian Air Force surveillance plane over the South China Sea in December.

There has been conjecture whether the government would conduct a more provocative "sail by" of reefs claimed by Beijing.

Responding to a question from a Chinese journalist at a conference in Washington last month, Foreign Affairs Minister Julie Bishop said Australia would continue "exercising our right to traverse the South China Sea in international water".

Australia has so far avoided a more overt and direct challenge within 12 nautical miles of artificial islands claimed by China.

Apocalyptic vision

During his heated 40-minute stump speech on Friday at a school hall in Derry, New Hampshire, Senator Rubio, warned of the dangers of Russian President Vladimir Putin's interference in eastern Ukraine and Syria, North Korea's "lunatic" with a nuclear weapons program and the "apocalyptic" Islamic State trying to "trigger the final end of the world showdown with the west".


"I can tell you the world is a more dangerous place than seven years ago," he said, in a dig at President Obama's global leadership.

He added: "When I am president we are going to have a real war on terror."

In a politically charged campaign speech, he also lambasted China's cyber theft of US intellectual property.

"The Chinese government is stealing our inventions and they have made billions of dollars off them," he said. "They hack our computers."

In Canberra, Senator Rubio is regarded as an internationalist. A well-known defence hawk, he favours a more muscular security strategy than Mr Obama, though is generally not viewed as a politician who would unnecessarily trigger military conflict.

His colourful rhetoric during a heated election battle, at a time when the American public has become more fearful of perceived national security threats, would probably be discounted to some extent in Canberra.

Chris Johnson, a former Central Intelligence Agency senior China analyst, recently told the Financial Review US-China tensions were likely to rise over the South China Sea and cyber security, and that would test Australia.

"Presuming the US-China relationship is entering some more difficult waters, the challenge for Australia is showing the consistency in the [US] alliance that they always do, while also doing what they have to do for their own economic security," he said.