WASHINGTON, April 20, 2004 — The number of people of Mexican ancestry in the United States grew by 53 percent in the 1990's, fueling much of the large overall increase in the nation's Hispanic population, new census data shows.

Mexicans are by far the largest Hispanic group in the United States, at 20.6 million, accounting for nearly 60 percent of the country's 35.3 million Hispanics. The next two largest groups are the 3.4 million Americans of Puerto Rican ancestry and the 1.2 million of Cuban ancestry.

But the fastest rate of Hispanic growth for the decade was among those who reported other origins, including Central and South America countries, a group that nearly doubled, to 10 million people in 2000.

The data released today by the Census Bureau paints in finer detail a statistical portrait of a Hispanic population that is growing not only in states with large Hispanic populations like California and Texas, but also in states like Georgia, Nebraska and North Carolina.

The growing number of Hispanics is a major reason that for the first time since the early 1930's, one of every 10 Americans is foreign born. It also explains why Hispanics have drawn about equal to blacks as the country's largest minority.

"The Latinization of the country is not just happening in New York, Miami or L.A.," said Juan Figueroa, president of the Puerto Rican Legal Defense and Education Fund. "Its greatest impact is in the heartland in places like Reading, Pa.; Lorain, Ohio; and Lowell, Mass."

The 2000 Hispanic total was about three million more than the Census Bureau had previously estimated, a difference that demographers attribute to a much better head count and far more undocumented immigrants than previously thought.

Half of the nation's Hispanic population lives in California and Texas, which helps explain why President Bush, a former governor of Texas, is negotiating with Mexico's president, Vicente Fox, to overhaul immigration and border policies.

The booming number of Hispanics, who can be from any race, carries sweeping implications for politicians. In Los Angeles, for example, Antonio Villaraigosa, the former speaker of the California Assembly, is hoping to ride the city's surging Hispanic population into office as its first Hispanic mayor since 1872.

In 122 of the 435 Congressional districts, Hispanics now exceed the national share of the Hispanic population, which is 12.5 percent, a new study by the National Association of Hispanic Elected and Appointed Officials has found.

Among these districts with significant Hispanic populations, 40 percent are represented by Republicans and 60 percent by Democrats, the study found.

"The political implications are that both political parties are rethinking their strategies not just in urban areas but other suburban areas," said Larry Gonzalez, the association's Washington director.

Hispanics now make up 42 percent of New Mexico's population and 32 percent of California's, and they are more than the national average in Arizona, Colorado, Florida, Nevada, New Jersey, New York and Texas.

A handful of cities in the West and Southwest, including East Los Angeles and Laredo, Tex., have populations that are more than 90 percent Hispanic. Puerto Ricans represented the largest Hispanic group in New York, at 36.5 percent; Mexicans had the largest Hispanic share in Los Angeles, at 63.5 percent.

The state with the smallest Hispanic population, the census found, was Vermont, with 5,504, followed by North Dakota, Maine and South Dakota.

The new census data also shows the relative youthfulness of the Hispanic population. While 26 percent of the United States population was under age 18 in 2000, 35 percent of Hispanics were under 18.

The rising Hispanic population also carries broad social implications, immigration experts said.

Steven Camarota, research director of the Center for Immigration Studies, a group that advocates more restrictive immigration policies, said poverty and welfare rates were higher among Hispanic immigrants.

But other research organizations, like the Tomas Rivera Policy Institute, have countered these observations with findings that show the Hispanic middle-class growing sharply over the last two decades.