Under questioning from Republican Senator Marco Rubio, a former Trump rival in the presidential race, Baker said the world "would be far less stable" with a weaker NATO or if more countries had nuclear weapons as Trump has proposed.
"We've a got a lot of problems today, but we'd have a hell of a lot more if that were the case," Baker told a Senate Foreign Relations Committee hearing, adding that US commitments around the world "promote US security."
Trump met with Baker on Thursday at Trump's request, said a Baker spokesman, who declined further comment.