No matter how the Bush administration and Congress act on illegal immigration in the US, any legislation or executive order is unlikely to answer the question: How many immigrants living in the country today are here illegally?
Depending on the source, the numbers range widely - from about 7 million up to 20 million or more.
Nailing down such figures is impossible. Even settling on a ballpark figure is difficult given the official sources: the US Census, apprehensions along the US-Mexico border, and social service agencies. For one thing, illegal immigrants avoid responding to census questionnaires, states a 2005 report by Bear Stearns Asset Management Inc. in New York.
Based on the national census in 2000, the US Census Bureau puts the estimate of illegal immigrants at 8.7 million. As of 2003, the US Citizenship and Immigration Services put the number at 7 million. Since then, United States immigration officials have said the number has grown by as much as 500,000 a year.
Those closest to the fight to protect US borders say the figure is higher. The US Border Patrol union Local 2544 in Tucson, Ariz., says the total number of illegal immigrants in the US today is between 12 million and 15 million.
The Pew Hispanic Center, a nonpartisan research organization in Washington, estimates 11.5 million to 12 million "unauthorized migrants" live in the US today. It bases its numbers on the "Current Population Survey," a monthly assessment of about 50,000 households jointly conducted by the US Bureau of Labor Statistics and the Census Bureau.
But in a letter to a constituent in 2004, Sen. John McCain (R) of Arizona wrote: "According to the US Border Patrol apprehension statistics, almost four million people crossed our borders illegally in 2002." Although many are caught and made to leave the country, a significant number try again. No one knows for sure how many succeed, but Senator McCain's assertion would mean that the number crossing the border and disappearing into the US economy could be much higher than official estimates.
"Deriving estimates of the number of unauthorized, or illegal, immigrants is difficult because the government lacks administrative records of their arrival and departure, and because they tend to be undercounted in the census and other surveys of the population," wrote the Congressional Budget Office in 2004.
Citing school enrollments, foreign remittances, border crossings, and housing permits, researchers at Bear Stearns reported "significant evidence that the census estimates of undocumented immigrants may be capturing as little as half of the total undocumented population."
There may be as many as 20 million illegal immigrants in the US today - more than twice the official Census Bureau estimate, according to Bear Stearns researchers Robert Justich and Betty Ng.
Looking at states where most of the undocumented population lives today, they reported "very dramatic increases in services required in communities that have become gateways for immigration." These include public school enrollment, language proficiency programs, and building permits. (California, Texas, Florida, New York, Illinois, New Jersey, Arizona, Georgia, and North Carolina account for about half the undocumented population.)
In addition, Bear Stearns found, these new undocumented workers are sending home significant amounts of money, suggesting that their numbers are considerably higher than official estimates. "Between 1995 and 2003, the official tally of Mexicans has climbed 56 percent, and median weekly wage has increased by 10 percent," the researchers found. "Yet total remittances jumped 199 percent over the same period. Even considering the declining costs of money transfers, the growth of remittances remains astounding."
One variable involves the relatives that join those coming across the border and form larger family units.
Nearly 14 million people (including 4.7 million children) live in "mixed status" families - in which the head of the household or the spouse is in the US illegally - Pew reported last summer. This is partly because children born in the US- regardless of their parents' legal status - are automatically US citizens.
"The large number of US citizen children born to parents with no legal status highlights one of the thorniest dilemmas in developing policies to deal with the unauthorized population," said Roberto Suro, director of the Pew Hispanic Center.
Whatever the total is, the annual number of illegal immigrants has exceeded those coming legally for at least the past 10 years: 700,000 illegally compared with 610,000 legally, according to Pew.
The number of "unauthorized migrants" (which includes some who have temporary permission to live in the US or those whose immigration status is unresolved), also has grown since legalization programs began in the mid-1980s, Pew reported last month: About 180,000 a year in the 1980s; 400,000 per year from 1990-1994; 575,000 per year from 1995-1999; and 850,000 per year from 2000-2005.
There's no doubt that Americans are concerned about the issue. "A growing number believe that immigrants are a burden to the country, taking jobs and housing and creating strains on the healthcare system," the Pew Hispanic Center wrote recently. "Many people also worry about the cultural impact of the expanding number of newcomers in the US."
Between 2000 and 2006, for example, the percentage of those polled who feel that immigrants are a burden because they take jobs and housing grew from 38 percent to 52 percent. At the same time, those who feel that immigrants "strengthen the US with their hard work and talents" dropped from 50 percent to 41 percent. In just the past 15 months, those who say "the growing number of newcomers from other countries threaten traditional American customs and values" has grown from 40 percent to 48 percent. Meanwhile, those who say newcomers "strengthen American society" has dropped from 50 percent to 45 percent.