Which doctrine provides congress with authority to regulate intrastate activities that substantially affect interstate commerce?

The answer is the dormant commerce clause. The Supreme Court helps impose the Commerce Clause by the "Dormant Commerce Clause", an implied restriction on the authority of State and Local governments to pass laws that restrict Commerce. This means that because Congress has been given power over interstate commerce, states cannot victimize in contradiction of interstate commerce nor can they unjustifiably burden interstate commerce, even in the nonappearance of federal legislation regulating the activity.