LAS VEGAS — Each morning, Israel Gonzalez rises before dawn and heads to the sidewalks around the city's plant nurseries to wait for a job. There, alongside other men, he watches for pickup trucks that slow down, hoping today he will be chosen for work.

It's a morning ritual played out regularly in cities and towns as day laborers, mostly illegal immigrants, scramble for work in a country that comfortably accepts their work while disavowing their right to be here.

The work is steady, the money is good, and when Gonzalez gets picked up for a job, no one asks for documents or identification.

"The bosses don't care if the papers are real or not," he said, wearing a navy hat with an American flag on it.

Gonzalez, 31, lives with his three brothers in an apartment; none of them is legal. They are among millions of illegal immigrants who work in obscurity, in the shadows of the American economy, quietly bringing home wages from people and companies more than willing to hire them.

On paper, many don't exist. Fake Social Security numbers and birth certificates make sure of that. They are nannies, housekeepers, landscapers, construction, farm and food-service workers. Cash is paid under the table, or fake documents are accepted without question.

Illegal immigrants may number as high as 20 million, and they are gaining a larger share of the job market, according to Bear Stearns in New York.

More and more, they are spreading beyond traditional immigrant states like California and Texas. They are spreading through the West and South, where there is tremendous growth, affordable housing and family networks. They are increasingly found in states such as Utah, Washington, Pennsylvania, Tennessee, Georgia and the Dakotas. And they're heading to suburbia.

This is America's underground economy, and it generates billions of dollars worth of labor each year. Illegal workers come for the jobs, and always find companies eager to hire them.

"The toleration of illegal immigration undermines all of our labor," said Vernon Briggs, a Cornell University labor economics professor.

"It rips at the social fabric. It's a race to the bottom. The one who plays by the rules is penalized. It becomes a system that feeds on itself. It just goes on and on and on."

For years, the immigrant population mainly stuck to six destination states — California, Texas, New York, Florida, Illinois and New Jersey. But in the past five years, the most rapid growth has taken place in states once of little interest to immigrants — Tennessee, Mississippi, the Dakotas, Ohio and Pennsylvania, said Bill Frey, demographer at the Brookings Institution in Washington, D.C.

The way Bob Justich sees it, America is hooked on cheap, illegal workers. As a senior managing director for Bear Stearns, he has spent the last two years meeting with immigrants, business owners, police and real-estate agents to determine the size of the underground economy and its effect on the real economy.

This he knows for sure: There are way more illegal immigrants in the country than the government estimates. The government puts the number at around 8.5 million; Justich says it is more than double that — closer to 20 million, mainly because illegal immigrants don't bother to respond to Census Bureau forms.

"If everybody was deported tomorrow, it would be like emptying the equivalent of New York state," he said. "And this source of labor has become vital to many businesses."

An analysis by Barron's estimated the size of the shadow economy at about $970 billion, or nearly 9 percent of the goods and services produced by the real economy.

The service sector employs the most illegal immigrants with 33 percent, followed by the construction industry, production and food-processing and farming, according to the Pew Hispanic Center.

The hotel and restaurant businesses and construction are the big employers. More than 1 of every 4 drywall installers and landscape workers are illegal, the center estimates. About 1 in 5 workers in meat and poultry packing are illegal, as are about 1 in 6 in the leisure and hospitality industry or construction.

Illegal immigrants make far less than the rest of the population. Their average family income of $27,400 is more than 40 percent below the legal immigrant or native family income of about $47,700, the Pew Hispanic Center found.

Enforcement is lax, especially in a post-9/11 world. The government doesn't have the time or resources to devote to rounding up illegal gardeners or maids; instead, it focuses on national security and critical infrastructure sites.

Nowhere is that more evident than in communities across the country where thousands of illegal immigrants wait for work on street corners. With the federal government paying little attention, many cities have been forced to create day-labor sites, where job seekers can congregate at a central location without loitering near businesses and bothering citizens.

That has come with its own set of problems. Critics don't believe local governments should use tax dollars to fund centers that cater to illegal immigrants.

In Herndon, Va., six residents, represented by the conservative legal group Judicial Watch, have sued the town over its plans to create a day-labor site. And in Farmingville, N.Y., Hispanics have been beaten, harassed and evicted.

"No one's solving the problem," said Wade Bohn, owner of Jay's Market, a gas station near where day laborers loiter in Las Vegas. The county here is considering creating a day-labor center.

"They're just moving it. Instead of enacting some type of legislation that forces them to become legitimate, they're trying to find a way to corral them and put them in a center."