The best succinct and accessible account of the theory of White Genocide that we have found is the Georgetown University Bridge Initiative’s Factsheet: White Genocide Conspiracy Theory. The contemporary formulation of the theory owes much to the “White Genocide Manifesto,” published from prison in 1995 by David Lane, a violent American white nationalist.
The Bridge Initiative notes a parallel with perceived threats to a more narrowly conceived white population a century ago. We found a clear account of the details of the earlier theory in Charles C. Alexander, Prophet of American Racism: Madison Grant and the Nordic Myth. The phrase “Anglo-Saxon or Nordic” more specifically denoted people of Germanic, Scandinavian, or British descent. Our observations on population figures come from the 1920 US Census (available on Wikipedia) and the 2010-2015 Census Bureau Community Survey.