He called going to the moon a mistake.
Why? I’m not an expert, but I’d guess that the moon could be mined for it’s resources and used as a launch base to go to Mars
He unfortunately proved to be a prophet.
He said the moon was too easy. We needed a sufficiently difficult goal to force the building of a large infrastructure that would become commercially self-sustaining. He was afraid that if we got to the moon, politicians, and to some degree the general public, would say, “ok, we did it, that’s that.”
And to a large degree that is exactly what happened. Politicians like Mondale went after NASA, claiming that the “money could be better spent on earth.” (Translation: welfare).