The US under a President Trump would withdraw Australia's security guarantee and become a "rogue superpower", according to a scholarly analysis commissioned by Australia's Lowy Institute.
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The results would be so dire that the US election is the most consequential for world order since World War II, and the most consequential for the US since the election of Abraham Lincoln in 1860, the report says.
"For the first time one of the two major party nominees for the presidency, Donald Trump, is campaigning on a platform of weakening the core elements of the international order, including the US alliance system and an open global economy," concludes Thomas Wright from the Brookings Institution in Washington.
He says Australia's alliance with the US would suffer directly as a result.
The Republican candidate has said he'd be prepared to re-negotiate or abandon US alliances with Japan, South Korea and NATO, but has not specifically mentioned Australia.
"Honestly, I think you are probably in the same category" as those US allies, Dr Wright told Fairfax Media. "He just hasn't been asked.
"He's said on the record that the US has no interest in being in Asia militarily. He doesn't think there's any intrinsic benefit in having alliances and Australia is in that category."
In his report, to be published on Monday, Dr Wright says: "The day after a Trump victory, allies would have to plan for the withdrawal of US security guarantees, rivals would look for ways to exploit the shift in US strategy, and markets would be rocked by the prospect of a new era of protectionism."
Dr Wright, the director of the Brookings Project on International Order and Strategy, also raises serious doubts about the US commitment to world order if Hillary Clinton won the election.
Dr Wright was an adviser to Barack Obama's campaign in 2008 and has donated to Mrs Clinton's campaign.
He projects that, although Mrs Clinton would be a president in the "traditional internationalist mode", a new isolationism would persist in the public mood.
"Even if Trump loses, this election has raised real doubts about whether the US will remain willing to serve as a liberal superpower on the world stage.
"For the first time since the second world war, a sizeable number of Americans have shown themselves to be receptive to a message of isolationism."
It's not just Mr Trump, but also the Democrats' runner-up, Bernie Sanders, who called for a cutback in US military and economic engagement with the world.
For a President Clinton, "her toughest mission will be to re-convince the American people as to the merits of increased US engagement to strengthen both the security and economic components" of the existing world order, according to Dr Wright.
He nonetheless expects that a President Clinton would resume her interest as Secretary of State in the US pivot to Asia, and that Australia and Japan are two allies that would receive more US attention in the process.
Dr Wright finds that, although Trump is not serious or consistent in many of his statements, he should be taken seriously on three key themes of foreign policy.
He describes "his three core beliefs that he has held for many decades and rarely if ever wavered from, the central themes of his campaign".
They are "opposition to America's alliance arrangements, opposition to free trade, and support for authoritarianism, particularly in Russia".
