This was a controversial proposition for two reasons. First, idealistic advocates of manifest destiny like John L. O'Sullivan had always maintained that the laws of the United States should not be imposed on people against their will. The annexation of "All Mexico" would be a violation of this principle. And secondly, the annexation of Mexico was controversial because it would mean extending U.S. citizenship to millions of Mexicans. Senator John C. Calhoun of South Carolina, who had approved of the annexation of Texas, was opposed to the annexation of Mexico, as well as the "mission" aspect of manifest destiny, for racial reasons. He made these views clear in a speech to Congress on January 4, 1848:
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.[47]
This debate brought to the forefront one of the contradictions of manifest destiny: on the one hand, while identitarian ideas inherent in manifest destiny suggested that Mexicans, as non-whites, would present a threat to white racial integrity and thus were not qualified to become Americans, the "mission" component of manifest destiny suggested that Mexicans would be improved (or "regenerated", as it was then described) by bringing them into American democracy. Identitarianism was used to promote manifest destiny, but, as in the case of Calhoun and the resistance to the "All Mexico" movement, identitarianism was also used to oppose manifest destiny.[48] Conversely, proponents of annexation of "All Mexico" regarded it as an anti-slavery measure.