Trump might invoke Article 2205 of NAFTA (on the mistaken assumption that tactics learned in shady New York property & labour negotiations serve his negotiating & domestic political standing) but this in & of itself does not withdraw the US from NAFTA as approval of Congress is needed to change tariffs and US laws that make NAFTA apply to US trade with Canada & Mexico.  

Canada & Mexico, in turn, might suspend current negotiations to reform NAFTA unless & until Trump agrees to bargain in good faith & without such over the top tactics.

The cumulative effect would be a standoff (one difficult to end y compromise) upsetting long range planning by companies & industries in all 3 countries, upset markets & trigger popular anger in both Mexico & Canada.

NAFTA would remain in force but all would suffer for months until this cloud is lifted by compromise.