The former prime minister John Howard says the difference between Prime Minister Malcolm Turnbull and his predecessor Tony Abbott is the way in which they present their policies.

Mr Turnbull cited a lack of economic direction as a reason for challenging Mr Abbott for the leadership of the Liberal party.

Mr Howard was asked on the ABC's 730 program what differences he saw between the Abbott and Turnbull governments' economic direction.

"There are obviously presentational differences, but there's a broad continuity," he said.

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"I don't want to get into the debate between Turnbull and Abbott. I'm not in the parliamentary party.

"My operating principle is to keep the Labor Party in opposition. I remind all Liberal supporters of the validity of that operating principle."

He said it was important to remember any government could be defeated at the polls — that the idea of unwinnable or unlosable elections no longer held sway.

"We've seen one-term state governments; we almost saw Tony Abbott tip out a one-term federal government in 2010," he said.

"We are seeing the mainstream political parties of the Western world fragmenting.

"You've got this extraordinary phenomenon in the US where it looks as though the Republicans are going to choose somebody who I think is manifestly unsuited for the job of president of the United States."

Mr Howard said a double-dissolution election was a possibility, but that Coalition supporters should keep in mind they tended to produce more minor parties than regular Senate elections.

He also said he understood the desire for Senate voting reforms.

"[But] people shouldn't lose sight of the fact that the principal beneficiary of those changes is probably the Australian Greens, and that is why the Australian Greens are so strongly in favour," he said.