Ontario’s Progressive Conservative Party is opposed to carbon taxes. We get it.

Premier Doug Ford ran for the party leadership last year on a promise to cancel the province’s cap-and-trade regime, and killing it was among the first things he did after he was elected.

That move triggered the federal carbon levy that came into effect on April 1, and Mr. Ford and his government have gone full bore in their attacks on it ever since.

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This week, the government is in court arguing against Ottawa’s power to impose a carbon tax on the four provinces – Ontario, Saskatchewan, Manitoba and New Brunswick – that have declined to create their own carbon-pricing regime.

On the Sunday before the federal tax took effect, PC caucus members posted co-ordinated tweets decrying it, accompanied by selfies of themselves gassing up their cars.

And last week, the province announced that it intended to put large stickers on gas pumps to remind drivers that the federal carbon tax is worth 4.4 cents a litre, an amount that will rise to 11.1 cents by 2020.

Up to a point, this is fair game. Carbon-pricing is a charged political issue in Canada, and Mr. Ford is on the side that says it is a “job-killer” and an ineffective way of fighting climate change. As Premier, he is free to use his elected platform to oppose the federal carbon tax as much as he wants.

But Mr. Ford crossed a line last week when his government introduced legislation that will oblige gas-station owners to put his anti-carbon-tax stickers on all their pumps, or face fines of up to $10,000 a day.

This is compelled speech, and it’s completely unacceptable. A government cannot force a business to become a medium for the transmission of a message that is self-evidently partisan.

How do we know the message is partisan? For one thing, Mr. Ford routinely disparages the federal levy as “the Trudeau Liberal carbon tax.” His own words give him away.

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As well, the stickers themselves, as currently conceived, are propagandistic half-truths that mimic Ontario PC rhetoric by conveniently telling only part of the story.

It’s true that the federal carbon tax has raised the price of a litre of gas in four provinces. But the PC Party and Mr. Ford, not to mention other conservative parties across the country, have consistently failed to acknowledge that Ottawa is returning the revenue it collects to the taxpayers of the provinces it came from. The stickers make no mention of this.

In fact, most Canadian households will get more back each year in carbon rebates than they will pay in carbon taxes. Ottawa will also use some of the revenues to provide support for schools, hospitals, universities and small and medium-sized businesses that will be paying more for fuel. And there is relief available to farmers, fishers, Indigenous people and people living in remote areas.

The whole point of carbon taxes is to use pricing to get industries and individuals to reduce their consumption of carbon-based fuels, either by cutting back or switching to alternatives. It is not, as conservatives across the country disingenuously claim, a money-grab.

Sometimes governments have good reason to compel companies to say something – to promote health and safety, for example. No one argues against the need to put up warning labels telling drivers not to light a cigarette while pumping gasoline.

And yes, governments of all stripes are guilty of buying advertising at taxpayer expense to push their party’s signature policies, all for partisan advantage. The federal Liberals recently sent postcards to millions of Canadians reminding them to apply for the carbon-tax rebate. This is a grey area that governments exploit, and they deserve criticism for it.

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But the Ford government’s move has no grey in it. The stickers are not about health or safety; they’re just an attack on the policies of another level of government. They’re nothing but a partisan poke.

The state should no more be able to force gas stations to display its propaganda than force homeowners to put anti-carbon-tax signs in the windows of their homes.

Conservative parties across Canada are waging a stubborn campaign of misinformation again the federal carbon tax. That is their murky prerogative. But when they try to use the weight of the law to force business owners to do their dirty work, that must be stopped.