Numerous US states count Canada as their biggest trading partner. Source: http://www.businessinsider.com/state-and-country-trade-maps-2015-7

Most of those states are led by Republican administrations.

Restricting the ability of those states to trade freely with Canada will hurt those Republican economies.

So relax. Before Trump is impeached - probably sometime in 2018 - it's highly unlikely he'll be allowed by Congress to damage those trade relationships.