The chief justice of Ontario's highest court got to the point early on Monday morning, one hour into arguments in the provincial government's case against the federal carbon tax.

Translating from legalese: Chief Justice George Strathy asked the lawyer for Ontario whether the courts couldn’t find some way for the province and the federal government to live and let live, as far as climate policy is concerned.

“There is a principle of interpretation that says courts should strive to find a harmonious interpretation of statute so they can live together, so you can have overlapping jurisdiction between provincial and federal legislation,” Strathy said to Josh Hunter, lawyer for Ontario’s attorney general. Shouldn’t it be possible to find a “harmonious interpretation” on carbon taxes?

The answer from Ontario, in short, is no: the province says the power the federal government is claiming is so far-reaching, so intrusive, that the usual rules of judicial peacemaking don’t apply. The core of Ottawa's carbon-tax legislation needs to rejected by the court, Ontario says, or the federal government will be able to trample all over the areas of provincial jurisdiction that are spelled out in the constitution.

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“It could impose restrictions on zoning; it could say you need a certain density of population on all new builds in the province,” said Hunter. “It could say, as some other countries have done, you need a licence-plate lottery, and you can only drive three days a week.”

This was a recurring theme: if the federal carbon tax is allowed, the courts will have given Ottawa the power to regulate nearly every sphere of human activity in Canada, as virtually everything has a climate impact.

This is the second time Hunter has had to make this argument in court — he made essentially the same arguments to the Saskatchewan Court of Appeal in February — and it likely won’t be the last. There’s a good chance there will be at least one more hearing: whoever loses in the opinion of this court will almost certainly appeal the decision of the five-justice panel to the Supreme Court of Canada.

Ontario’s fundamental argument against the federal government is relatively straightforward, at least as far as constitutional law is concerned (the day’s arguments included the words “but there’s an exception to the exception”). Ottawa is claiming that the power to regulate and tax greenhouse-gas emissions is a matter of “national concern,” a phrase with specific meaning in Canadian constitutional law. If a matter is of national concern, the federal government has the power to intervene — even if that means treading into areas generally reserved for the provinces. But to win this argument, Ottawa would need to prove that the problem is so big that the provinces wouldn’t be able to solve it on their own collectively or individually.

Canada will present its side on Tuesday, but it filed its primary arguments with the court earlier this year, and, like Ontario’s, they mirror the ones made in Regina in February.

It’s not all constitutional law and high principle, of course. The federal carbon tax has emerged as a political lightning rod in Ontario, Saskatchewan, and other provinces. There’s a provincial election in Alberta today, and the United Conservative Party — whose leader, Jason Kenney, is the odds-on favourite to become Alberta’s next premier — is scheduled to make arguments to the Court of Appeal tomorrow.

The federal Liberal party has embraced the fight over climate change as it tries to shift attention away from the first three scandal-plagued months of 2019. Getting to fight over something relatively uncontroversial (to Liberal and Liberal-leaning voters, at least) like climate change instead of something brutal and divisive like the SNC-Lavalin controversy probably comes as something of a relief.

Even if the Conservative, Progressive Conservative, and United Conservative parties lose these court challenges, they’ll be able to tell supporters that they did their best — and, incidentally, the carbon tax can still be beaten at the ballot box in October.

But for the Liberals, the risk is more serious. Courts can be unpredictable, and the federal government is defending its signature climate policy on two fronts. It’s not unthinkable that one or both provincial courts could rule against the government. And then what happens?

It’s one thing to tell voters that only the Liberals stand between hard-working Canadian families and climate-change oblivion. Voters who, rightly, fear the future that unmitigated climate change will bring may reasonably swallow other disagreements with the Liberals — even large ones — if they think the incumbent party is the best bet to address one of their core priorities.

But what will those voters do if, after swallowing disappointments on numerous other files, the highest court in the country’s largest province says the feds can’t even get their climate policy right?

Arguments at the Court of Appeal in downtown Toronto continue all week.