Mary Waters, a sociologist at Harvard University, remembered being stunned when she saw the research.

“It was like, ‘Oh wow, these nerdy projections are scaring the hell out of people,” she said.

Beyond concerns about the data’s repercussions, some researchers are also questioning whether the Census Bureau’s projections provide a true picture. At issue, they say, is whom the government counts as white.

In the Census Bureau’s projections, people of mixed race or ethnicity have been counted mostly as minority, demographers say. This has had the effect of understating the size of the white population, they say, because many Americans with one white parent may identify as white or partly white. On their census forms, Americans can choose more than one race and whether they are of Hispanic origin.

Among Asians and Hispanics, more than a quarter marry outside their race, according to the Pew Research Center. For American-born Asians, the share is nearly double that. It means that mixed-race people may be a small group now — around 7 percent of the population, according to Pew — but will steadily grow. Are those children white? Are they minority? Are they both? What about the grandchildren?

“The question really for us as a society is there are all these people who look white, act white, marry white and live white, so what does white even mean anymore?” Dr. Waters said. “We are in a really interesting time, an indeterminate time, when we are not policing the boundary very strongly.”

The Census Bureau has long produced projections of the American population, but they were rarely the topic of talk shows or newspaper headlines.