在美國歷史上所簽訂的 14 項 FTA 中，NAFTA 與 CAFTA-DR 最能展現總統與國會在 FTA 政策制訂上所扮演角色與影響決策的程度，故本文以此兩項 FTA 作為研究美國 FTA 決策主體的案例。研究發現，在對外政策的決策主體的三種不同的模式:總統主導模式、國會主導模式與互動模式中，總統模式最能解釋美國 FTA 政策。總統具有龐大的國內外行政資源與政黨力量，可以干預國會的決策並改變個別議員的投票傾向，在不影響 FTA 基本內涵與目的下促使國會通過 FTA。國會制訂的程序規範不足以限制行政部門的權力;除了最終的投票權，國會的影響力僅存在於洽簽時介入修改某些條款，而這些條款的改變仍需要行政部門的認可。這種由行政部門主導，國會僅能在不影響 FTA 根本的前提上修改部分的條款，明顯的符合總統主導模式。

Among all 14 FTA’s signed by the U.S. in history, NAFTA and CAFTA-DR - selected by this paper as cases to identify the entity that owns the power of dominating the U.S. FTA policy - most represent the dynamics and impacts of President and Congress on the FTA policy formation. The research shows that among the three models of decision-making entities of foreign policy - President, Congress, and Interaction, the President model best explains the decision-making of FTA policies. President controls enormous external and internal administrative resources and party leverage to interfere with the Congress decision-making and change the voting behavior of individual congressman, resulting in the passage of FTA’s in Congress without undermining its foundation. Regulations created by Congress do not generate meaningful effects on constraining President’s power. In addition to the showdown on final voting, Congress’ influence is only limited to slightly modifying contents of agreements mostly in the stage of negotiation, which also requires approval from the Administration. This is apparently consistent with the arguments of the President model.