There was an "All-Mexico" movement in the US both in congress and among average Americans advocating for the annexation of Mexico. However, many American politicians were uncomfortable with the idea of annexing so many people who would resist US law and occupation. Many southerners, in particular, simply did not like the idea of incorporating non-whites into the union. Senator John C. Calhoun, in particular, said:

We have never dreamt of incorporating into our Union any but the Caucasian race—the free white race. To incorporate Mexico, would be the very first instance of the kind, of incorporating an Indian race; for more than half of the Mexicans are Indians, and the other is composed chiefly of mixed tribes. I protest against such a union as that! Ours, sir, is the Government of a white race.... We are anxious to force free government on all; and I see that it has been urged ... that it is the mission of this country to spread civil and religious liberty over all the world, and especially over this continent. It is a great mistake.