Canadian environmental groups have been quiet this election. Too quiet.

The reason, three environmental charities tell VICE News, is because the Canada Revenue Agency (CRA) audited them last year and warned them against appearing partisan. Now, even with climate change and pipelines on the agenda this election, they're afraid of being too outspoken.

In previous elections, some environmental groups would issue report cards on each party's election platform — but they are not doing that this election because, according to the CRA, ranking candidates or parties amounts to favoring one over another.

One group told VICE News they were instructed not to use the words "Harper government" in any of their public statements because that could be construed as partisan.

Under the rules, Canadian charities can spend up to 10 percent of their time on political activities — but they must be non-partisan. Running afoul of these rules could lead them to lose their charitable status. The environmental charities VICE News contacted were so fearful of backlash, they wouldn't speak on the record.

But well-known environmentalist David Suzuki, who resigned from his foundation's board so he could speak his mind, didn't hold back.

"There's been a huge chill across the environmental sector because of the threat," Suzuki said over the phone Thursday. "The cost of a single audit is huge, and for a lot of organizations it's almost crippling," he said.

The David Suzuki Foundation is currently in its third audit, with a cost of over $100,000 per audit, he estimated.

"Virtually all of the major environmental groups have been audited," he continued. "So it's huge, and you are more careful, you are less outspoken if you are audited or have the threat of an audit. There's no question it's really quieted the opposition."

In 2012, the Conservative government tightened the rules around how charities report their political activities, and gave the CRA an extra $8 million over two years to change its reporting form and audit charities for compliance.

Last year, the CRA began in-depth audits of major environmental charities for their political activities, including Environmental Defence, Tides Canada, Equiterre, the Ecology Action Centre, the Pembina Foundation, and the David Suzuki Foundation. In the case of West Coast Environmental Law, the agency flew in their "A team" from Ottawa for the audit, CBC reported. And earlier this year, the Sierra Club Canada Foundation was audited for political activity in what its director called an "intimidation campaign."

Some of these audits, including those for Environmental Defence, the David Suzuki Foundation, and Tides Canada, were driven by complaints from Ethical Oil, a registered non-profit NGO. In the case of Environmental Defence, Ethical Oil has argued, "Charities are supposed to do work for the public good, like feeding the poor or funding research to cure AIDS. Taxpayers should not be subsidizing this lobby group's partisan actions and political agenda."

Audited charities VICE News spoke to said they were happy to comply with the guidelines — but they thought the rules were being applied unevenly to target environmental groups specifically.

One charity gave the example of the Munk Debates, the charitable group that held a federal election debate but did not allow Green Party leader Elizabeth May to take part — a move May complained was partisan.

Their concerns echoed those voiced by the left-leaning Broadbent Institute in late September. A report by the institute concluded that major right-leaning Canadian charities reported zero political activity to the CRA, even though they are allowed to dedicate up to 10 percent of their revenue toward political efforts.

"This report makes it clear that the CRA rules around political activity are interpreted, to put it charitably, quite differently by many right-leaning charities," the institute said.

National Revenue Minister Kerry-Lynne Findlay has said there is no bias in how officials choose charities for audits.

Since he stepped down from the board, Suzuki has been able to criticize political parties and candidates. "I can't see how you can possibly discuss the campaign if you don't start naming actual people," Suzuki said.

In August, a group of at least 11 environmental and public policy organizations, including some that had been audited, asked the five major federal parties whether they would support "a new legal and policy direction that enhances and protects the ability of registered charities to participate in public policy debates."

The Bloc Quebecois and the Conservatives did not respond to their request.

The Green Party said they supported "revising and updating the laws relating to charitable NGOs, so that NGOs can participate in advocacy and be able to retain their charitable status and their ability to accept tax-deductible donations."

The Liberals said they would do "a significant overhaul" of CRA practices, including "ending the CRA political harassment of charities," and supporting the role of charities to advocate for public policy.

The NDP said they would end "the campaign of politically motivated audits of Canadian charities," and update laws to allow charities to participate in advocacy and debates of public policy.

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