Amid much optimism the UN declared last week the world was on course to pass the threshold necessary to put the Paris climate change deal into effect. More than 55 countries have joined the agreement, representing about half of all global emissions. Yet that milestone could almost instantly be undone if the US courts reject President Barack Obama’s clean energy plan.

A group of Republican states and energy companies say Mr Obama’s methods, which aim to cut US carbon emissions by more than a quarter within a decade, are unconstitutional. The hearings start on Tuesday. At question is the courts’ view of the scope of Washington’s regulatory powers. At stake is US leadership on climate change — and the viability of the Paris deal itself.

The objections are deceptively abstruse. Opponents of Mr Obama’s plan say he has overstepped his powers by requiring US states to cut their carbon emissions. They claim this over-interprets the Clean Air Act, which gives Washington’s Environmental Protection Agency the power to regulate local pollutants, such as mercury and sulphur. They also say Mr Obama’s plan violates the US constitution which bans the federal government from taking control of state resources.