Final answer:
The most advantageous school of thought for U.S. foreign policy is a pragmatic combination of realism, liberalism, and constructivism. The President should consult Congress, interest groups, and allied countries to gain support for foreign policy initiatives.
Step-by-step explanation:
The question of which school of thought the United States should follow in foreign policy is complex and heavily depends on the global and domestic context. However, one could argue that a pragmatic approach that incorporates elements from various schools of thought, such as realism, liberalism, and elements of constructivism might be most advantageous. These approaches respectively acknowledge the importance of power, the role of international institutions and law, and the influence of ideas and identities in shaping international affairs.
As for which actors to approach to gain support for a new foreign policy initiative, the President would be wise to consult with members of Congress, particularly those in leadership positions or on relevant committees, to secure legislative backing. Additionally, reaching out to influential think tanks and interest groups, as well as getting input and support from allied countries with a stake in the policy, would be essential for creating a broad coalition of support.