NAFTA members are prohibited from imposing tariff on goods imports. A small 15% tariff can be imposed for a short duration. Anything more will violated the agreement. This can trigger NAFTA lawsuits. 

To make Trump's threat stick he have to impose much more than 15% and for a long time, at least 5 years. 
NAFTA lawsuits are notorious - they are run by lawyers and penalty hard to impose. But Trump wanton violation of NAFTA will make an easy quick win. The tariff will be rolled back and reimbursed. Severe penalty imposed. It will be messy but in the end Trump's tariff treatment will fail. So he may just cancel NAFTA and impose still tariff. 

The damages done to the U.S. will be severe because no one in the world will strike trade agreement with the U.S. anymore. Same result if Trump tears up NAFTA. Much of the world continue to benefit from preferential trade while the U.S. won't. The U.S. dollar will collapse. After all, isn't the U.S. petrol dollar a form of preferential trade?