Massive street protests have catapulted immigration to the top of the US agenda, pitting workers against business interests. But a crackdown on illegal workers could create huge economic strains for all.
Proponents of a guest worker programme, including President George W. Bush, argue that the mostly Hispanic immigrants fill a labour shortage. Opponents, including some labour unions, say they take jobs from Americans, drive down wages, strain social services and increase poverty.
Economists say it is impossible to gauge the exact impact of an undocumented workforce intent on staying in the shadows. Still, many analysts say driving illegal immigrants out of the workforce, as proposed by some conservative lawmakers, would strain labour markets at a time when the unemployment rate is already at a four-and-a-half-year low.
"Rounding up millions of workers and sending them back to Mexico is going to be more disruptive to the economy than not, so it makes sense to move them into the mainstream," said Jeff Faux, of the left-leaning Economic Policy Institute.
According to the Pew Hispanic Centre, more than seven million of the estimated 11.5 million to 12 million unauthorised migrants in the United States are employed.
That means about one in 20 US workers is illegal, and the share is even higher in specific industries: about 24 per cent of workers in farming, 17 per cent in cleaning, 14 per cent in construction and 12 per cent in food preparation.
With US unemployment at 4.7 per cent, officials at the Federal Reserve and White House have said the economy is near full employment - the point at which the scarcity of job applicants threatens to spark wage inflation.
John Gay, senior lobbyist at the National Restaurant Association, said competition for workers in many industries is already stiff.
"You can't open a restaurant... unless you have a full staff in the back of the house - dishwashers and prep cooks and busboys, jobs that have typically been filled by immigrants in the industry," Mr Gay said.
But while the unemployment rate is low, there are still seven million unemployed Americans and while the two groups do not all compete for the same work, the unemployment rate for those without a high school diploma is much higher than the national average, at 7.0 per cent.
Brookings Institution immigration scholar Audrey Singer said the least-educated and poorest Americans are most hurt by illegal immigration.
"Take the poultry and meat packing industries. These used to be primarily native-born just 10, 12 years ago, and in some places... they are now heavily foreign-born," Ms Singer said.
American University history professor Alan Kraut said immigrants have always been more welcomed by employers than by the population at large.
"Immigrants keep the wage scale down, you can pay them less than the minimum, they won't squawk, they'll be a disciplined workforce because they can be fired or turned into immigration authorities. It's an employer's dream," Mr Kraut said.
Mr Faux is unsympathetic to concerns on Wall Street or at the central bank that wage increases could spark inflation.
"Yes, if you stopped the flow of immigrants, that certainly would make wages go up. What's wrong with that?" he said.
The Fed has raised interest rates 15 times since mid-2004 and sharp wage growth would likely prompt even higher borrowing costs. Some Fed officials have worried aloud about the economic impact of a backlash against illegal immigrants.
"It's not my job as a central banker to get involved in the emotion of this argument, that is our political leaders' responsibility. But I pray that they will keep the economic aspects of this firmly in mind," Dallas Fed President Richard Fisher said last week.
Ms Singer said today's costs of illegal immigration, in low wages, overcrowded school and health care, will be outweighed in the long run by the benefits of having taxpaying immigrants when an aging America needs them most.
"Paying for it now is going to pay off later," Ms Singer said.