Whites already are a minority. They have been for decades. Only 30 percent of North Americans are a genetically pure anything.
The small political constituancy of those who refuse to disavow parts of their heritage in favor of one is growing.
Where did you obtain your statistic? It is my understanding that 20% of African Americans have white ancestry and 30% of White Americans have a African American Ancestor. Of course, this dose not take into account Indian or Asian ancestry, but even if included, I doubt it would be as low as 30 percent.