Former PM John Howard says Donald Trump is 'not compatible with ...

Exclusive: John Howard says he could not vote for Donald Trump, declaring the former US president's refusal to accept the 2020 election had effectively made him unfit to contest the US election.

But the former prime minister has stopped short of endorsing Democrat presidential candidate Kamala Harris, saying he had not been impressed by her economic message.

On Trump, Howard was scathing, saying his behaviour had not been "compatible with democracy".

"In normal circumstances, I would unhesitatingly favour a Republican victory, but there are reasons that prevent me doing that on this occasion," Howard said in an interview with 9News.

The former PM used a cricket analogy to explain his distaste for Trump's behaviour after the 2020 presidential election.

"I think Donald Trump should have left the field when the umpire's finger went up," he said.

"In other words, I think his refusal to accept the result of the last election and various attempts to overturn that result not compatible with democracy.

"When you play the democratic game, you've got to accept the democratic result."

Former US vice president Dick Cheney, a lifelong Republican whom Howard knows well, last week endorsed Harris, saying Trump could "can never be trusted with power again".

Cheney asserted Trump had attempted to "steal the last election using lies and violence to keep himself in power after the voters had rejected him".

Howard said he shared that sentiment "in the abstract" and said he did not envy American voters on November 5.

"The choice the American people must make is a difficult choice - I'm glad I don't have a vote there, because I think it would be very hard," Howard said.

"I'm not impressed with Kamala Harris. I don't think she's got a very good idea of how to run the American economy."

He added: "I worry that she might lead an administration that will spend far too much, and some of that expenditure will be irresponsible ... she leads a party that, generally speaking, favours far too much government intervention for my liking."

Howard said political events seen in the US showed its system had major deficiencies.

"Everything we're witnessing in America tells us is that their political system is far inferior to ours," he said.

"In a parliamentary system, Donald Trump would never have got to the leadership of the Republicans, and I don't think Kamala Harris would ever have got to the leadership of the Democrats.

"We wouldn't have gone through the agony the Democrats went through regarding Joe Biden.

"They have a different system. And my take, my very strong view, is that a parliamentary system is much better than a presidential system."