Several U.S. senators grilled trade czar Jamieson Greer on tariffs Wednesday after U.S. President Donald Trump lobbed new threats at Canada.

Those threats loom large as Canadian premiers meet in Charlottetown, in part to calculate next steps.

The president plans to impose 50 per cent levies on a range of Canadian goods in retaliation against various provinces’ bans on selling U.S. liquor – itself a response to Trump’s trade regime and annexation threats. The president also cited Canada’s protections on its domestic dairy industry.

Unlike other tariffs imposed on Canadian goods, these duties have no exemptions for goods protected by the Canada-U.S.-Mexico Agreement on trade, better known as CUSMA. They are set to begin on Aug. 19.