In the 1940 US census, "white" included anyone of Mexican descent.
President Franklin D. Roosevelt promoted a Good Neighbor policy that sought better relations with Mexico. In 1935, a federal judge ruled that three Mexican immigrants were ineligible for citizenship because they were not white, as required by federal law.
Mexico protested, and Roosevelt decided to circumvent the decision and make sure the federal government treated Hispanics as white. The State Department, the Census Bureau, the Labor Department, and other government agencies therefore made sure to uniformly classify people of Mexican descent as white.
This policy encouraged the League of United Latin American Citizens in its quest to minimize discrimination by asserting their whiteness.
The race category of "Mexican" was eliminated in 1940, and the population of Mexican descent was counted with the white population.
What started as Slaves OR Free Whites OR Other Free People later granulated in fair arbirtray and often political ways: https://upload.wikimedia.org/wikipedia/commons/f/ff/USCensus...
The entire "science of race" in the US was fairly arbitrary phrenology adjacent quack science.