The Canadian and Ontario governments square off this week in the province's top court over Ottawa's climate-change law.
Experts say the case is as much political and ideological as it is legal.
At issue is the validity of federal legislation that imposes a carbon tax on gasoline and other fossil fuels.
Ontario has denounced the law as an illegal tax grab and is challenging its constitutionality.
Among other things, the province argues Ottawa is intruding on provincial jurisdiction.
Ontario Premier Doug Ford is supported by like-minded counterparts in Manitoba, Saskatchewan and New Brunswick.
The proceedings in Ontario's Court of Appeal will be livestreamed with cameras allowed in the courtroom for the first time in more than a decade.