Senior MPs say the party must unite and can do so only if support for the scheme is scrapped.

''Unity trumps policy,'' one frontbencher told The Sun-Herald.

Leadership speculation has swung away from moderate frontbencher Joe Hockey to conservative Tony Abbott, although neither wants to challenge Mr Turnbull.

Mr Abbott was previously pushing for the party to do a deal with the Government in order to put the issue to rest. But he now believes climate change has become such a toxic issue it could split the party.

Mr Turnbull yesterday refused to comment on leadership speculation but continued to mount the case for an emissions trading scheme.

''Our amendments seek to preserve tens of thousands of … jobs and to enable an emissions trading scheme to be much more effective,'' Mr Turnbull said. ''Whether we're able to achieve that will depend on the Government's reaction [and] we won't know finally what that is until the negotiations are concluded.''

Negotiations between the Government and the Opposition are scheduled to end tomorrow.

Assistant Climate Change Minister Greg Combet said it was difficult to negotiate with a divided Opposition. ''The disunity of the Coalition is still the major barrier to … [getting] the legislation through.''

The Government has already caved in to Opposition demands that agriculture be permanently excluded from the scheme and agreed to allow farmers to make money through initiatives such as offering land for carbon offsets.

The bulk of the Opposition's wish list - including more compensation for heavy-polluting industries - remains unresolved. Sources said the party room would not accept anything less than close to 100 per cent capitulation to their demands.

''If you had a house on the market for $400,000 you'd take $380,000 but you wouldn't take $250,000,'' one party source said.

The Government will demand a vote on the scheme late this week. Defeat would give it the trigger for a double-dissolution election.

The Government has also been warned it faces a bill of billions of dollars if it tries to strengthen the scheme in later years.

Lawyers for the Greens said any toughening of the scheme would hit taxpayers, who would be forced to cover the cost of government buying back carbon permits. ''All Kevin Rudd has given … is spin, reciting climate rhetoric while presenting a polluter reward system that will fail the climate and the community,'' Greens senator Christine Milne said.