That's signaling intention to uphold NATO by that particular congress, not something that would prevent a US government from bringing it down. If a US president

That's signaling intention to uphold NATO by that particular congress, not something that would prevent a US government from bringing it down. If a US president were only to make it known that he or she would actively resist giving the required orders to fulfill NATO commitments in a crisis, I think the alliance in its current form would be walking dead from that point forward. There is considerable public doubt already as to whether the US would honor their commitment to wage a nuclear war over NATO matters and that's the keystone of the whole alliance. If Russia were to attack a NATO member and win the gamble, I'd expect a massive shift of power on the continent and a great loss of US soft power in Europe (at the least).