The Hispanic and Asian-American populations in the United States are expected to triple by 2050, when non-Hispanic whites would account for the barest majority, according to a Census Bureau report to be released Thursday.

Hispanic-Americans would make up nearly a quarter of the nation's population at mid-century, the report says.

"This is going to be the workforce that sustains us as a nation, so we can make choices today that are dramatically going to change the outlook 20 or 30 years from now," Sonia Perez of the National Council of La Raza said Wednesday, referring to coming national elections.

The number of Hispanic-Americans should rise to nearly 103 million from about 36 million, and their share of the population would nearly double, to 24.4 percent from 12.6 percent, the bureau report says.

Asian-Americans, who now make up 3.8 percent of the populace, would represent 8 percent by mid-century, it says. Their numbers would increase to more than 33 million from nearly 11 million.

The American population over all should also continue to grow, to about 420 million in 2050 from 282 million in 2000, the report says. But non-Hispanic whites would add only moderately to their numbers, to 210 million from 196 million. They would make up just 50.1 percent of the population in 2050, compared with 69.4 percent four years ago, when the last census was taken.

The black population is projected to rise to 61 million from 36 million, raising its share of the total population to 14.6 percent from 12.7 percent.

The American population will be generally older, with child-bearing rates remaining low as those born between 1946 and 1964, the baby boom generation, begin to turn 65 in 2011. By 2030, about one American in five will be 65 or older.

As the baby-boom generation moves toward retirement and seeks payments from Social Security, the largest source of new workers will be young Hispanics, said Roberto Suro of the Pew Hispanic Center.

"Financing the old age of the baby boomers would be more difficult without a growing Hispanic population," Mr. Suro said.

Over all in the United States, women will continue to outnumber men, with 6.9 million more women than men in 2050, compared with 5.3 million more in 2000.