WASHINGTON » The nation’s minority population is steadily rising and now makes up 35 percent of the United States, advancing an unmistakable trend that could make minorities the new American majority by midcentury.

As white baby boomers age past their childbearing years, younger Hispanic parents are having children—and driving U.S. population growth.

"The aging of baby boomers beyond young middle age will have profound impacts on our labor force, housing market, schools and generational divisions on issues such as Social Security and Medicare," said William H. Frey, a demographer at the Brookings Institution. "The engine of growth for the younger population in most states will be new minorities."

New census estimates show minorities added more than 2 percent in 2009 to 107.2 million people, boosted by an increase in Hispanic births and more people who described themselves as multiracial. During this time the white population remained flat, making up roughly 199.9 million, or 65 percent, of the country.

By comparison, whites comprised 69 percent of the total population in 2000, and minorities 31 percent.

Four states—Hawaii, New Mexico, California and Texas—as well as the District of Columbia have minority populations that exceed 50 percent. That is one state more than in 2000, when Texas was not on the list.

About 311 of the 3,143 counties—one in 10—have minority populations of 50 percent or greater. That’s up from about 250 counties in 2000.

The census estimates released yesterday documented a widening age and race divide. They are the last government numbers before completion later this year of the 2010 census, which could change the balance of political power when legislative districts are redrawn based on population and racial diversity.

A key factor in the demographic transformation is aging baby boomers, a predominantly white group now shepherding college kids instead of starting young families. Since 2000 the number of whites under age 45 decreased by 8.4 million, while the number of whites over that age rose by 12.6 million.

The result is that the number of white younger adults and children fell in 42 states. Fifteen states, led by California, New York, Pennsylvania and Michigan, have lost more than 10 percent of their younger white population since 2000.

Locally the changing race dynamics were widespread.

Seven U.S. counties saw their minority populations become the majority last year: Gwinnett County, Ga.; Titus and Victoria counties in Texas; Finney County, Kan.; Saguache County, Colo.; Contra Costa County, Calif.; and Yakima County, Wash.

The rise in the minority population is due to recent sharp increases in minority births, especially among Hispanics, who accounted for more than half of total U.S. population gains last year. There are now roughly 9 births for every 1 death among Latinos, compared with a roughly 1-to-1 ratio for whites.

Based on current rates, data from the 2010 census could show a new "tipping point" in which babies born to minorities outnumber babies born to whites.

"Fertility is playing a critical role in reshaping the racial and ethnic structure of the country," said Kenneth Johnson, a sociology professor at the University of New Hampshire.

Multiracial Americans, the fastest-growing U.S. demographic group, are also adding to minority gains. About 5.3 million were identified last year as being of multiple race or ethnicity, up 3.2 percent from the previous year.

Among racial and ethnic groups, Hispanics grew by 3.1 percent to 48.4 million, and Asians increased 2.5 percent to 13.7 million. They now represent about 15.8 percent and 4.5 percent of the U.S. population, respectively.

Blacks, who make up about 12.3 percent of the population, increased less than 1 percent last year to 37.7 million.

The 2009 Census estimates used local records of births and deaths, tax records of people moving within the U.S., and government statistics on immigrants. The figures for "white" refer to those whites who are not of Hispanic ethnicity.

Results from the official 2010 head count will be published beginning in late December.