Donald Trump had shown himself to be a "fascist threat" who posed the gravest risk to the United States since that country's Civil War, a top-ranking US Democrat warned in Sydney on Thursday.

Martin O'Malley​, a one-time governor of Maryland and former entrant in this year's presidential contest, said it was encouraging that Hillary Clinton was now doing better in the race to the White House in the aftermath of the first two debates against Mr Trump.

Former Maryland governor Martin O'Malley. Credit:AP

But he said "the threat has not passed. Democracy is always a fragile thing. Perhaps it was hubris for us to think that we were so much better than Germany in 1936."

Mr O'Malley said there remained a "profound sense of alienation" among many American voters who had seen their wages flat-line or decline over the past decade, and it was startling that Mr Trump had not yet been relegated to "slinking along in single digits" in the polls.