Tony Abbott.

www.tonyabbott.com.au

FEDERAl Liberals MPs are airing their allegiances as the clock ticks down on what could be Malcolm Turnbull's final 24 hours as opposition leader.

Tony Abbott, the only declared challenger, says he is not overly confident of his chances in a leadership ballot on Tuesday, frontrunner Joe Hockey has yet to make up his mind, while Kevin Andrews won't rule out another tilt at the top job.

Mr Turnbull is facing a second leadership spill in less than a week, as the Senate continues debating Labor's carbon pollution reduction scheme.

Mr Abbott arrived at Parliament House on Monday saying he was not "over confident" he had the numbers.

"I'm still talking patiently and carefully to all of my colleagues," he told reporters.

Mitch Fifield, one of more than a dozen Liberal MPs to resign from the frontbench or other senior positions last week, said the Liberals had already moved on from Mr Turnbull.

"On Tuesday we will have a new leader of the party," he said, while backing Mr Hockey as the replacement.

An opinion poll, published on Monday, shows the opposition's treasury spokesman has the edge over Mr Turnbull with voters.

At least two Liberals, chief climate change negotiator Ian Macfarlane and senator Gary Humphries, declared they were in Mr Turnbull's corner.

And former opposition leader John Hewson, rolled as party leader in 1994, said the moves against Mr Turnbull were both a tragedy and gross disloyalty.

He also warned Mr Hockey that he risked ending his political career "early" if he accepted the leadership.

But Howard government foreign minister Alexander Downer said he thought Mr Hockey would be a good choice.

"He was a very good minister and handled some difficult issues. He's a charming guy as you can see just from watching him on television, a very entertaining and amusing person, a very decent person," Mr Downer told Fairfax Radio.

"I think if he does become the leader he would be a very interesting choice."

Meanwhile, the Senate has begun what could be a marathon sitting as it considers more than 200 amendments to Labor's legislation.

Rebel coalition senators will attempt to refer the legislation to a Senate committee, delaying any final vote until February.