There is one man who will decide whether Malcolm Turnbull hangs on or whether Peter Dutton becomes Australia's 30th prime minister.

It is Mathias Cormann.

The Belgian-born Finance Minister and Government leader in the Senate is the sole buttress supporting the Turnbull prime ministership.

As soon as he walks, the PM's castle collapses.

On Wednesday he made a clear statement that he is standing by his man.

"I have served Malcolm loyally ever since and I will continue to serve him loyally into the future," he said.

Liberals estimate that if Senator Cormann does shift, he would bring 10 votes to the Dutton column.

Senator Cormann has been conspicuously quiet in recent days. He is usually the person dispatched at times of trouble for the Government.

But his public appearances have been few and far between … until Wednesday.

His personal angst is high. He has desperately sought to make the Turnbull administration work. He has negotiated legislative victories with patience and renowned courtesy. He and Mr Dutton have been Mr Turnbull's conservative Praetorian Guard.

Together they gave Mr Turnbull's prime ministership a conservative veneer.

Senator Cormann wanted to push ahead with the company tax plan, long after colleagues believed the agenda politically cooked. Defeat came today in the Senate for that plan. A rare defeat for the man they call the Cormannator.

But Mr Dutton is his best mate.

The WA Senator did not vote for Mr Dutton yesterday. He couldn't. Not in his leadership position.

But his colleagues believe this may be about to change. And if it does, it is game over for the Prime Minister.

Is Mathias Cormann about to switch his allegiance to best mate Peter Dutton? ( ABC News: )

Liberals say Mr Turnbull needed two-thirds of the Tuesday leadership vote to remain safe. He did not get it. The seven-vote margin is simply not enough and minds are already turning to whether Mr Dutton could make a successful transition to leader.

His detractors, including Nationals frontbencher Darren Chester, say Mr Dutton would be unelectable in the southern states.

But Mr Dutton does have his backers well south of Gympie.

One Liberal figure says his very ordinariness would be an asset in the way John Howard's was; that as a plain-speaking copper in an egalitarian Australia he would ring truer than Turnbull ever has.

Like Senator Cormann, Mr Dutton is a pragmatist with Paul Keating-like cut-through, one Liberal figure said.

"Australians don't like pretension and with Dutton it could work. He could be believed when it comes to feeling the pain of the lower class and middle classes on electricity prices," he said.

Many Liberals remain doubtful about Mr Dutton's capacity to make that transition.

Mr Turnbull's fate depends on whether Senator Cormann is one of them.