The danger is less that Trump takes the U.S. out of NATO or withholds nuclear deterrence thus minimizing the risk for Russia to attack a Europan NATO state. More dangerous is a U.S. military attack e.g. on Canada which will create an Article 5 incident that leads to the break-up of NATO from within.

(The North Atlantic Treaty (1949) has no provision for such an incident that member states of the alliance turn violently on each other. It was an unthinkable possibility in the times of Roosevelt and Truman but is now easily imaginable. A post-Trump NATO will have to amend the Treaty.)