WASHINGTON - Multiracial Americans have become the fastest-growing demographic group, wielding an impact on minority growth that challenges traditional notions of race.

The number of multiracial people rose 3.4 percent last year to about 5.2 million, according to the latest census estimates. First given the option in 2000, Americans who check more than one box for race on census surveys have jumped by 33 percent and now make up 5 percent of the minority population - with millions more believed to be uncounted.

Demographers attributed the recent population growth to more social acceptance and slowing immigration. They cited in particular the high public profiles of Tiger Woods and President Obama, who are having an effect on those who might self-identify as multiracial.

Population figures as of July 2008 show that California, Texas, New York, and Florida had the most multiracial people, due partly to higher numbers of second- and later-generation immigrants who are more likely to "marry out." More than half of the multiracial population is younger than 20 years old.

The latest demographic change comes amid a debate on the role of race in America, complicating conventional notions of minority rights. Under new federal rules, many K-12 schools next year will allow students for the first time to indicate if they are "two or more races."

The move is expected to cause shifts in how test scores are categorized, potentially altering race disparities and funding for education programs.

Five justices of the Supreme Court have signaled they would like to end racial preferences in voting-rights and employment cases. Blacks and Hispanics, meanwhile, tout a growing minority population and past discrimination in pushing for continued legal protections. The Hispanic growth rate slowed last year to 3.2 percent.

Left out of the discussion are multiracial people, who are counted as minorities but can be hard to define politically and socioeconomically. Demographers say that while some multiracial Americans may feel burdened or isolated by their identity, others quickly learn to navigate it and can flourish from their access to more racial networks.

"The significance of race as we know it in today's legal and government categories will be obsolete in less than 20 years," said William H. Frey, a demographer at Brookings Institution. "The rise of mixed-race voters will dilute the racial-identity politics that have become prevalent in past elections."