Constitutionally, Congress controls tariffs. Congress chose to transfer that power to the President in dozens of statutes over the centuries. The Trade Expansion Act empowers Trump to use tariffs whenever needed for national security. The International Emergency Economic Powers Act empowers Trump to use tariffs in any emergency. The Trade Act empowers Trump to use tariffs to protect US industry and offset injurious foreign trade practices.
Trump’s tariff policy is both Constitutionally rooted and Congressionally approved, with a sound basis in national security and productive economy, reflecting a broader view of the purpose of the economy to make Americans more secure in all respects from foreign threats. As such, it conforms directly to the beliefs and intent of the founding generation.
Robert Barnes
"In the discussions to which this interest has given rise and in the arrangements by which they may terminate the occasion has been judged proper for asserting, as a principle in which the rights and interests of the United States are involved, that the American continents, by the free and independent condition which they have assumed and maintain, are henceforth not to be considered as subjects for future colonization by any European powers. "
-- James Monroe, speech to the US Congress on December 2, 1823