“In fact, we don’t want them to make cars for us. We want to make our own cars. We don’t need their lumber. We don’t need their energy. We don’t need anything from Canada. And I say the only way this thing really works is for Canada to become a state.”

Article 1, Section 9 of the US Constitution:

No Tax or Duty shall be laid on Articles exported from any State.

Trying to remove any and all trade barriers in order to prevent trade.