The president can't impose tariffs - Congress has to do that.
I would be willing to WAGER that in NAFTA and all these other sell out trade deals there are MANY PROVISIONS the Administration can UTILIZE to Impose Higher Costs, Extremely Higher Costs, CONGRESS already gave the store away and with it their power to regulate.
There seems to be disturbing trend of people wanting Trump to just act unilaterally. We already have that, so substituting one emperor for another, isn't very appealing.
Actually, according to the Reciprocal Tariff Act of 1934, the President has a large amount of power in dealing with tariffs....
https://en.wikipedia.org/wiki/Reciprocal_Tariff_Act