WASHINGTON: New census data released on Thursday affirm a clear and sustained drop in illegal immigration, ending more than a decade of increases. 

The number of illegal immigrants in the US dropped to an estimated 11.1 million last year from a peak of 12 million in 2007, part of an overall waning of Hispanic immigration.

For the first time since 1910, Hispanic immigration last year was topped by immigrants from Asia.

Demographers say illegal Hispanic immigration – 80 per cent of all illegal immigration comes from Mexico and Latin America – isn't likely to approach its mid-2000 peak again, due in part to a weakened US economy and stronger enforcement but also a graying of the Mexican population.

The finding suggests an uphill battle for the Republicans, who passed legislation in the House last week that would extend citizenship to a limited pool of foreign students with advanced degrees but who are sharply divided on whether to pursue broader immigration measures.

In all, the biggest surge of immigration in modern US history ultimately may be recorded as occurring in the mid-1990s to early 2000s, yielding illegal residents who now have been settled in the US for 10 years or more. They include migrants who arrived here as teens and are increasingly at risk of “aging out” of congressional proposals such as the DREAM Act that offer a pathway to citizenship for younger adults.

“The priority now is to push a vigorous debate about the undocumented people already here,” said Jose Antonio Vargas, 31, a journalist from the Philippines.

“We want to become citizens and not face the threat of deportation or be treated as second class,” said Vargas, whose campaign, Define American, along with the young immigrant group United We Dream, have been pushing for citizenship for the entire illegal population in the US.

The groups point to a strong Latino and Asian-American turnout for President Barack Obama in last month's election as evidence of public support for a broad overhaul of US immigration laws.

Earlier this year, Obama extended to many younger immigrants temporary reprieves from deportation. But Vargas, who has lived in the US since 1993 and appeared this year on the cover of Time magazine with other immigrants who lacked legal status, has become too old to qualify.

“This conversation is a question about how we as a nation define who is an American,” Vargas said, noting that if politicians don't embrace immigration overhaul now, a rapidly growing bloc of minority voters may soon do it for them.

“If you want us to pay a fine to become a citizen, okay. If you want us to pay back taxes, absolutely. If you want us to speak English, I speak English. But we can't tread water on this issue anymore.”

Jeffrey Passel, a senior demographer at the Pew Research Center and a former Census Bureau official, said US immigration policies will have a significant impact in shaping a future US labour force, which is projected to shrink by 2030.

Aging white baby boomers, many in specialised or management roles, are beginning to retire. Mexican immigration, which has helped fill needs in farming, home health care and other low-wage US jobs, has leveled off.

“Immigration is one way to boost the number of workers in the population,” he said, but the next wave of needed immigrants is likely to come from somewhere other than Mexico. “We are not going to see a return to the levels of Mexican unauthorized immigration of a decade ago.”

The numbers are largely based on the Census Bureau's Current Population Survey through March 2011. Because the Census Bureau does not ask people about their immigration status, Passel derived estimates on illegal immigrants largely by subtracting the estimated legal immigrant population from the total foreign-born population.

The numbers are also supplemented with material from William H. Frey of the Brookings Institution and Mark Mather of the Population Reference Bureau, who reviewed data released Thursday from the Census' American Community Survey.

The data showed that 11.1 million, or 28 per cent, of the foreign-born population in the US consists of illegal immigrants, virtually unchanged since 2009 and roughly equal to the level of 2005.

An additional 12.2 million foreign-born people, 31 per cent, are legal permanent residents with green cards. And 15.1 million, or 37 per cent, are naturalised US citizens.

Fewer Mexican workers are entering the US, while many of those immigrants already here are opting to return to their homeland, resulting in zero net migration from Mexico.

In 2007, legal and illegal immigrants made up equally large shares of the foreign-born population, at 31 per cent, due to ballooning numbers of new unauthorised migrants seeking US construction and related jobs during the mid-2000s housing boom. Naturalised US citizens then represented 35 per cent.

Broken down by geography and race, roughly half of all states last year posted declines or no change in their numbers of foreign-born Hispanics, including big immigrant states such as California and New York as well as economically hard hit areas in Arizona, Georgia and North Carolina, which previously had seen gains.

Foreign-born Asians were a bigger source of population gain than Hispanic immigrants in California, New York, Virginia, Illinois and New Jersey. Newly moving into suburban communities, the Asian population spread out more across the southeastern US and Texas, increasing their share in 93 per cent of the nation's metropolitan areas.

As a whole, foreign-born residents are slowly graying, with 44 per cent now age 45 or older. They are more likely than in 2007 to be enrolled in college or graduate school (39 per cent, up from 32 per cent) and to be single (17 per cent married, down from 22 per cent).

Births to immigrant mothers also are on the decline, driving the overall US birth rate last year to the lowest in records dating back to 1920.

“At least temporarily, the face of immigration to the US is changing in terms of cultural background, education and skills,” Frey said. “The fertility bump provided by past Hispanic immigrants may not be replicated in the future, especially if Asians take over a greater share of US immigrants.”

House Republicans, seeking to show they are serious about addressing the immigration issue after being largely rejected by Hispanics in the election, voted last week to make green cards accessible to foreign students graduating with advanced science and math degrees from US universities.

The measure, strongly backed by the high-tech industry and touted as a boost to the US economy, would have a net effect of extending more visas and eventual citizenship to students from India and China.

It is opposed by most Democrats, the Obama administration and immigrant rights groups such as the Asian American Justice Center which want to see it packaged with broader legislation that extends legal status for illegal immigrants.

These groups also oppose the proposed new 55,000 visas for foreign students because they would be offset by eliminating a lottery program that provides green cards to people with lower rates of immigration, mainly those from Africa. Senate Democrats on Wednesday blocked Republicans from bringing up the bill.

A bill introduced by Sens. Jon Kyl of Arizona and Kay Bailey Hutchison of Texas, who are retiring at the end of this session, seeks to offer some legal status to young immigrants.

Critics say it falls short because it does not provide a path to citizenship, an issue that Sen. Robert Menendez, D-N.J., describes as “non-negotiable.”

About 77 per cent of Hispanic voters in the November election said they thought people working in the US illegally should be offered a chance to apply for legal status, according to exit polling conducted for the television networks and The Associated Press. That is compared with 71 per cent of Asian-Americans and 65 per cent of voters overall.

The political implications are great.

Hispanics and Asian-Americans are the nation's two fastest-growing population groups, each increasing by more than 40 per cent since 2000. A higher birth rate and years of steadily high immigration have boosted Hispanics to 17 per cent of the US population, compared with blacks at 12 per cent and five per cent for Asians.

Even if the nation's estimated 11 million illegal residents do not attain citizenship, the nation's Hispanics, who made up roughly ten per cent of voters in November, are expected to nearly double their share of eligible voters by 2030. Asian-Americans, who now are three per cent of voters, will also continue to increase.

About 73 per cent of Asian-Americans voted for Obama, second only to African-Americans at 93 per cent and slightly higher than Latinos at 71 per cent, according to exit polling.

Asian-Americans don't strongly identify with either party, but they tend to cite jobs, education and health care as issues most important to them and generally prefer a big government that provides more services.

Relatively new to the US and religiously diverse, Asian-Americans also may have been repelled by Republican Mitt Romney's forceful stance during the primaries seeking “self-deportation” of immigrants as well as the GOP's sometimes narrow appeal to evangelical Christians, said Karthick Ramakrishnan, a political science professor at the University of California-Riverside who helps conduct a broad National Asian American Survey.

While Mexicans make up about 55 per cent of illegal immigrants and other Latin Americans represent another 25 per cent, Asians make up a ten per cent share, many of whom overstay temporary visas.