The 1930 Census race or color instructions for “Mexican” show a clear connection with the theme of racial intermediacy and ambiguity:
all persons born in Mexico, or having parents born in Mexico, who were not definitely White, Negro, Indian, Chinese, or Japanese.
(Instructional language quoted from D’vera Cohn.)
Third, the U.S. Census had always counted persons of Mexican descent as whites, except in 1930, when a special category was created for “Mexicans.” The question of Mexican racial identity became especially acute during the immigration restriction debates of the 1920s. This broad exemption from immigration quotas led to the historic congressional debates in the 1920s by restrictionists determined to close the door to Mexicans. The Bureau of the Census decided that beginning with 1930 it would establish a new category to determine how many persons of Mexican descent resided in the United States, legally or illegally. Before 1930 all Mexican-descent people were counted simply as white persons, because the racial categories at that time included Negro, White, Indian, Chinese, and Japanese. The 1930 census  created, for the first time in U.S. history, the separate category of “Mexican,” which stipulated that “all persons born in Mexico, or having parents born in Mexico, who are not definitely white, negro, Indian, Chinese, or Japanese, should be returned as Mexican.” This meant that census workers determined whether to record a particular Mexican household as “white” or “Mexican.” About ninety-six percent of Mexican-descent people were counted under this new category of mexican; only four percent were counted as white. Mexicans had, for the first time in U.S. history, been counted as a nonwhite group. The government of Mexico as well as numerous Mexican Americans protested this new classification. Bowing to pressure, the U.S. government abandoned the category of Mexican in the 1940 census but sought other means of identifying the Latino population, by identifying those with Spanish surnames or households whose dominant language was Spanish.
Neil Foley, "Partly Colored or Other White: Mexican Americans and Their Problem with the Color Line," in Beyond Black and White: Race, Ethnicity, and Gender in the U.S. South and Southwest
, ed. Stephanie Cole and Alison M. Parker, 123-144 (College Station: Texas A&M University Press, 204), 129-130.