Cordelia Scaife May, an heiress to the Mellon family’s banking and industrial fortune, was far and away the most important donor to the modern anti-immigration movement during her lifetime. Now, more than a decade after her death, her money still funds the leading organizations fighting to reduce migration.

Her Colcom Foundation has poured $180 million into groups that spent decades agitating for policies now pursued by President Trump: militarizing the border, capping legal immigration and prioritizing skills over family ties for entry. And language she used — about the threat of an “immigrant invasion,” for instance, and environmental strain — echoes in today’s anti-immigration rhetoric, most recently in the words of the killer in the El Paso mass shooting.

The New York Times unearthed letters from Mrs. May and other personal writings that reveal what motivated the movement’s publicity-shy benefactor.