Final answer:
The majority of the US supported the president in the Mexican American War due to the military victory, perceived justifications, and implications for the slavery debate.
Step-by-step explanation:
The majority of the US supported the president in regard to the Mexican American War for several reasons. One reason was the war's military victory, which gave the American military valuable experience and prestige. Additionally, many Americans believed that the war was justified and necessary to protect American interests. Lastly, the war also had implications for the ongoing debate over slavery, with some supporting the war as a means to expand slavery. Overall, these factors contributed to the support for the president during the Mexican American War.