The rapidly changing face of the American people is revealed in new census projections that predict that groups that are now labelled minorities will form the greater part of the country's population by 2042.

According to the US census bureau, the dominance of non-Hispanic white people, who today account for two-thirds of Americans, will be whittled away, falling steadily to less than half in 2042 and 46% by 2050. In the opposite trajectory, those who describe themselves as Hispanic, black, Asian and Native American will increase in proportion from about a third now to 54% by 2050.

Such a rapid demographical shift is in tune with trends that have been seen for some time, but it is happening much faster than experts had predicted even four years ago.

Demographers see it as among the most intensive changes of a country's racial and ethnic make-up in history, every bit as dramatic as the huge influx of Italian, Irish and east European immigrants that transformed the US in the early 20th century.

"The new projections move up some of the changes that are happening. The minority population now will become the majority eight or nine years earlier than we had thought," said David Waddington, who heads the census bureau's population projections team.

The shift in majority status from non-Hispanic whites, who have enjoyed the dominant position since European settlers from England, the Netherlands and elsewhere overtook the Spanish and Native American population centuries ago, is likely to have profound implications. In the long term, it could prompt a sea-change in the country's understanding of its politics and culture. More immediately, it is likely to inform debates on immigration policy and reshape the electoral landscape.

Two underlying factors are strongly at play in the new projections. The first is that the overall population of the country is growing faster than expected. It passed the 300 million-mark in October 2006, and is projected to cross the 400 million landmark by 2039. By 2050 it will reach 439 million.

That extraordinary rate of growth is in turn fuelling the second main underlying factor, which is the huge increase in the US Hispanic population. In the census bureau definition, Hispanic refers to a person's description of their origin in places such as Mexico or South America, rather than their race.

The projections suggest that the Hispanic population will increase from 15% of the population today to almost a third by 2050, almost tripling in size from 47 million to 133 million. By contrast, the non-Hispanic white population is expected to remain relatively steady numerically, barely rising from 200 million to 203 million. For the first time in US history, the white population will lose numbers between 2030 and 2040.

Of the other main groups, the black population is projected to remain relatively stable in proportional terms, rising from 14% of the population today to 15% in 2050, from 41 million to 66 million.

The Asian population is projected to climb more sharply, from 16 million, or 5%, to 41 million, or 9%.

The impact of the changes is likely to be further magnified by the seismic change in the country's age profile that will occur simultaneously. As the baby-boomer generation passes into older age it is causing an extreme bulge, like the shape of a snake's torso after eating a rabbit. Over-65s are 12% of the US population this year, but by 2050 they will comprise one in five Americans.

Cross-tabulate those figures with the racial and ethnic composition of the nation and the results are even more striking. The growth in minority groups is heavily focused on younger age groups. They already account for 45% of American children under 18, and that proportion will rise to 62% by 2050.

"It means there's going to be a young-old divide," said William Frey, a demographer at the Washington-based Brookings Institution. Frey points out that young adults aged 18 to 29 are already the most ethnically and racially diverse, and as a group have tended to back Barack Obama in this year's election. By the presidential election of 2028, Hispanic, black and Asian people will be in the majority within that age range.

"But the over-65s will primarily remain white and will stay that way. The challenge for any national politician will be appealing to such diverse interests at both ends of the spectrum."

The census bureau points out that its projections are based on assumptions about future births, deaths, immigration from abroad and internal migration that it has extrapolated from trends over the past 20 years. Those factors could change, particularly the rate of immigration, which is already a matter of heated debate across the country.