Australia has "no choice" but to look beyond Washington for security as Americans become increasingly isolationist, an expert warns.

An annual report from the US Studies Centre has revealed Americans are less certain about the value of international partnerships, and most now believe their country's best days are behind it.

The Canberra Times reports that USSC chief executive Simon Jackman warned the country's fractious politics dwarf international affairs in the minds of its voters, a trend likely to worsen if Donald Trump launches another tilt at the presidency.

But Jackman predicted growing isolationism in the US means Australian diplomats face an uphill battle regardless of who sits in the White House.