WINNIPEG - A carbon tax would be good for the planet and for Canadians' wallets, Liberal Leader Stephane Dion said Tuesday as he pitched his soon-to-be-released green plan.

But Prime Minister Stephen Harper quickly came out swinging, dismissing the idea as "insane.''

Dion is expected to unveil a proposal Thursday to fight climate change with a "green tax shift'' that would put a price on carbon, including the use of oil and gas.

"The basic idea is, that to fight climate change, you need to put the price on greenhouse-gas emissions, especially carbon dioxide,'' Dion said in a speech to the Canadian Club.

"When it's free, it's not effective as a plan''.

Dion repeatedly emphasized that a Liberal carbon tax wouldn't cost taxpayers a cent, because it would be revenue neutral, offset by tax cuts.

"We need to make polluters pay, and put every penny back into the hands of Canadians.''

Dion said the process would be reviewed annually by the auditor general to ensure it was revenue neutral, adding it wouldn't require more administration.

He pointed out that in British Columbia, which introduced a carbon tax this year, not a single new civil servant had to be hired to carry out the program.

It's one of the advantages of using a tax shift instead of a cap-and-trade system, said Dion.

"The cap and trade is a regulation. And the way the Conservatives are trying to prepare their own regulation on climate change, it's very cumbersome and it will be very bureaucratic, and a nightmare for a lot of business people.''

The Conservatives have been attacking Dion's plan for weeks as a tax grab _ even though the details haven't been released yet. They kept up the assault Tuesday in the House of Commons.

"Now the Liberal party wants to raise taxes across the board as part of its insane environmental and economic policies,'' Harper said. "That is wrong for Canada.''

Environment Minister John Baird said he doesn't believe the Liberal plan would be revenue neutral.

"This tax will be revenue neutral is like, `The cheque is in the mail.' It's like, `I'll respect you in the morning.' It's just not believable.''