Most adult Mexicans are voters by the organic law; but few take measures to make use of the right. … They are regarded by slaveholders with great contempt and suspicion, for their intimacy with slaves, and their competition with plantation labor.
Americans, in speaking of them, constantly distinguish themselves as “white folks.” I once heard a new comer informing another American, that he had seen a Mexican with a revolver.  “I shouldn’t think they ought to be allowed to carry fire-arms. It might be dangerous.” “It would be difficult to prevent it,” the other replied; “Oh, they think themselves just as good as white men.”
Frederick Law Olmsted, A Journey Through Texas; or, a Saddle-Trip on the Southwestern Frontier. New York: Dix, Edwards & Co, 1857. 163-164.
[context: long section devoted to “The Mexicans in Texas.” Most seemingly based on travel through Bexar (?)]