Opposition Leader Tony Abbott has warned the Labor Party it would be "committing suicide twice" if it tried to block his efforts to repeal the carbon tax.

Labor says it will not "cave in" to the Coalition's moves to replace the tax with its Direct Action policy if Mr Abbott wins Saturday's poll, adding currency to the Opposition Leader's threat to hold a double dissolution election next year.

Mr Abbott is standing firm, again indicating that he will retain the option of calling a double dissolution election on the issue.

"A Labor Party which persists in support of the carbon tax is just setting itself up to lose not one election but two," he said.

Key points: Tony Abbott says Labor would lose a double dissolution if it blocks repeal of carbon tax

Tony Abbott says Labor would lose a double dissolution if it blocks repeal of carbon tax Mr Abbott reiterates view that election is 'referendum' on the issue

Mr Abbott reiterates view that election is 'referendum' on the issue Labor's Mark Butler says party will not 'cave in' to Coalition demands

Labor's Mark Butler says party will not 'cave in' to Coalition demands Labor says Direct Action policy 'a complete dud'

Labor says Direct Action policy 'a complete dud' Greens pledge to hold both major parties to account

He added that "no sane political party" would stand by a tax that had become such "electoral poison".

"If the Coalition wins the election on Saturday, the carbon tax will go," he said.

"No ifs, no buts, it is gone.

"We will do whatever is necessary to abolish the carbon tax.

"I tell you this, if we win the election, which is a referendum on the carbon tax, the last thing the Labor Party will do is commit political suicide twice by continuing to support this absolutely toxic tax."

But Prime Minister Kevin Rudd has said "absolutely" carbon pricing is an article of faith for the ALP.

"Carbon pricing is fundamental to how you deal with climate change," he said.

"Any alternative response to that is just intellectually dishonest."

Mark Butler says Labor won't cave in on carbon tax

Earlier, Environment Minister Mark Butler said Labor would maintain its long-held support for an emissions trading scheme and would not support Direct Action.

"This mandate argument that says that if you lose an election, hypothetically, you junk all of your long standing policy positions and simply cave in to the government of the day is an utter fallacy," he told Radio National Breakfast.

"It's never been that way, and frankly it never should.

"Why have an opposition if you're not going to have opposition MPs argue strongly for policy platforms that you took to the election?

"And if we happen to lose on Saturday that is what the Labor party will be doing on climate change, and on industrial relations and on a range of other core policy positions that we've had for years."

Labor seizes on Tony Abbott's Direct Action comments

Yesterday, in his National Press Club speech, Mr Abbott conceded the Opposition Direct Action policy may not succeed in cutting greenhouse gas emissions by the stated aim of five per cent by 2020.

He said a Coalition government would not spend more than what has been budgeted, which on its 2010 document was $3.2 billion over four years.

"We will get as much environmental improvement as much emissions reduction for the spending that we've budgeted," he said.

Labor says it has shown up the policy for what it really is.

"The Direct Action policy is a complete dud," Mr Butler told RN.

"Experts have now been saying for months that the only way Tony Abbott could actually achieve the sort of reduction in carbon pollution he's signed up to is by spending literally billions and billions more than he's budgeted."

Greens pledge to hold both parties to account

Greens leader Christine Milne says if her party retains the balance of power in the Senate, it will try to hold both major parties to account.

"Tony Abbott has walked away from any pretence that he takes global warming seriously," she told ABC Radio News.

"He once referred to climate change as crap and it's pretty obvious that that is what he thinks.

On the ALP, she added that "we will be able to shame the Labor party in opposition into holding the line".

The option to call a double dissolution election arises when a particular piece of legislation is rejected twice by the Senate, with a three-month gap in between the votes.