Obviously it would be best if trump didn’t say something so unhinged but no one thinks trump is inviting Russia to invade.
Nato is undermining itself by doing nothing and putting their entire security burden on one country in a completely different continent. Trump is threatening nato. He is saying you better carry your weight or we may not save you.
I would argue that trumps threat should strengthen nato because if he is elected then his stance puts pressure on nato members to meet their obligations. Nato members meeting their obligations makes nato stronger. If nato members are worried about getting invaded and having no support from the us there is a simple solution contribute 2% gdp to military spending. If nato members are so worried about an invasion then they should probably participate in their military alliance.