Speaking on Saturday at a rally in South Carolina, Mr Trump recalled how as president he told a Nato country that he would “encourage” Russia to do as it wishes in cases of Nato allies who are “delinquent.”
“You didn't pay? You're delinquent?”’ he recounted saying. “'No I would not protect you. In fact, I would encourage them to do whatever the hell they want. You gotta pay. You gotta pay your bills."'
Reacting to his comments, Nato Secretary-General Jens Stoltenberg said the 31 allies remained committed to defending each other.
"Nato remains ready and able to defend all allies. Any attack on Nato will be met with a united and forceful response,” he said.
“Any suggestion that allies will not defend each other undermines all of our security, including that of the US, and puts American and European soldiers at increased risk.”
Mr Stoltenberg added that he expects that, “regardless of who wins the presidential election, the US will remain a strong and committed Nato ally.”
Mr Trump’s remarks will concern Nato countries in Europe threatened by Russian aggression, including Poland and Latvia.
During his presidency, Mr Trump threatened to not to come to the aid of any country under attack that he considered to not be spending enough on defence.
Under Nato’s mutual defence clause, called Article 5, all allies commit help of any member who comes under attack.
The White House described Mr Trump’s comments as “appalling and unhinged”.
“Encouraging invasions of our closest allies by murderous regimes endangers American national security, global stability and our economy at home," White House spokesperson Andrew Bates said in a statement on Saturday night.
"Rather than calling for wars and promoting deranged chaos, President Biden will continue to bolster American leadership and stand up for our national security interests - not against them."