In a sure sign that politics makes strange bedfellows, Conservative Leader Stephen Harper and NDP Leader Jack Layton both took aim at Liberal Leader Stephane Dion's "Green Shift" carbon tax plan Thursday.

Harper lashed out at the plan, saying it would cause a catastrophic recession and fracture national unity.

"The purpose of their plan is not to lower emissions, it's to get money for the government, and that will cost every consumer," Harper said at a Montreal campaign stop.

Layton attacked Dion for the first time on the campaign trail, calling the Liberal plan "bad policy" and said the NDP's cap-and-trade plan for reducing emissions would be a more effective system than a carbon tax.

But Matthew Bramley, of the sustainable energy think-tank Pembina Institute, said the two leaders' criticisms of the carbon tax plan is not backed up by the facts.

"Mr. Harper and Mr. Layton's opposition to carbon taxes is contradicted by leading economics and business organizations who say taxing pollution is a good way to harness market forces to fight global warming," Bramley said in a statement.

"There is no evidence to support Mr. Harper's claim that a modest carbon tax would cause a recession. In fact, Mr. Dion's proposed tax would need to be further increased to enable Canada to meet science-based targets for greenhouse gas reductions."

Both Norway and Sweden have had a carbon tax plan since the early 1990s.

According to an April 29, 2008 Guardian article, Sweden cut its overall carbon emissions by nine per cent between 1990 and 2006. Its economy grew by 44 per cent in that period.

Dion's "Green Shift" plan would tax fossil fuels, returning the costs in the form of income tax cuts.

The Liberals describe it as revenue and cost-neutral -- something Harper has dismissed as unrealistic.

John Williamson of the Canadian Taxpayer's Federation said on CTV Newsnet Thursday that Dion's plan was not truly "revenue neutral."

"This is a fiscal transfer from taxpayers to low income Canadians," he said. "That is not the definition of a tax neutral plan."

Another type of criticism

Harper used his strongest language yet to slam the Liberal leader's plan, saying it would fan separatist flames by putting too much power and money back in Ottawa's hands.

"The carbon tax will do more than undermine the economy," Harper said. "By undermining the economy and by re-centralizing money and power in Ottawa, it can only undermine the progress we have been making on national unity."

Harper said his party is the only one with a realistic election platform, suggesting the other parties would all either have to boost taxes or run a deficit to pay for the promises they are making on the campaign trail.

Dion defends plan

Dion, who began Day 5 of the campaign in New Brunswick, tried to sell the $15 billion plan to supporters at a pancake breakfast.

Many Liberals in the Maritimes have complained the plan is hard to sell to residents in provinces where much of the industry relies heavily on fuel, such as forestry, fishing and oil refinement.

He told the Saint John Board of Trade that Harper's criticism of the plan doesn't hold water.

"Stephen Harper said his plan for the country is to 'stay the course.' But we know the course he set in 2006 -- he has allowed what was a booming economy to hit a brick wall," Dion said.

"Even Statistics Canada admits that the struggling U.S. economy is outperforming Canada's economy. Statistics Canada reported yesterday that Canada's labour productivity has fallen for nine months straight. This is Canada's worst performance since 1990 - since Brian Mulroney."

He also criticized Harper, saying he is trying to turn every election issue into a debate on national unity.