Which antebellum-era writer and intellectual described the Mexican war as a "dose of arsenic" to the American body politic?

Ralph Waldo Emerson described the Mexican War, and the acquisition of formerly Mexican territory, in this way. Of course, he turned out to be correct--the debate over the expansion of slavery into the territories formerly controlled by Mexico unleashed political forces that ultimately tore the country apart. So this quote is often cited as an example of Emerson's prescience. But interestingly, as historian George Herring has observed, Emerson wasn't really referring to the issue of...

Ralph Waldo Emerson described the Mexican War, and the acquisition of formerly Mexican territory, in this way. Of course, he turned out to be correct--the debate over the expansion of slavery into the territories formerly controlled by Mexico unleashed political forces that ultimately tore the country apart. So this quote is often cited as an example of Emerson's prescience. But interestingly, as historian George Herring has observed, Emerson wasn't really referring to the issue of slavery. Rather, he viewed the Mexican people as unable to function within American (by which he meant white) democratic society. Emerson, like many intellectuals of his time, tended to conflate race and culture, and he believed that the Mexican people, who he identified with Roman Catholicism and despotism (also conflated in the minds of many Americans) would undermine American democracy. This, of course, is astonishingly ironic from a historical perspective, but it was a powerful argument against Manifest Destiny (and one which would later be made against imperialism). When Emerson said "Mexico will poison us," he meant the Mexican people themselves, not the political debate over slavery that actually ensued.