PHOENIX - The United States needs not only the undocumented workers already here but even more foreigners to do jobs, the head of the nation's largest business group said Wednesday.

Thomas Donohue, president and chief executive officer of the U.S. Chamber of Commerce, said immigrants, legal and otherwise, account for almost half of the labor in farming, fishing and forestry. He said they make up a third of everyone working in building cleaning and maintenance, a quarter in construction and close to 20 percent in health care.

And Donohue said there also are immigrants in skilled occupations.

"Stop and think what would happen to key industries if we rounded up and sent home 12 million undocumented workers that are in this country today,'' he told a group of business people. "The las one out could turn off the lights.''

But even with that, he said the country can't fill all of the jobs. Donohue said the United States is at "nearly full employment,'' saying the 4.7 percent national jobless rate includes about 3 percent he termed "unemployable.''

The only solution, he said, is more foreign labor. And that, said Donohue, means changing the laws to make it easier for those workers to get here and, in some cases, to stay.

"A fundamental purpose in creating an effective, rational and national immigration system should not be to stop the flow of immigrants to our country but rather to continue it and, I'm sorry to say to some people, to expand it because we're going to need more and more workers as our economy grows and grows,'' he said.

Donohue also insisted the shortage of workers in some industries would not be cured by companies offering higher salaries. "You can't pay more to workers who don't exist in the first place,'' he said.

But he conceded immigration can affect wages.

Figures from the state Department of Economic Security show that hourly wages in Arizona went up 12.3 percent between 2001 and 2006. But among roofers the wage hike was just 8.9 percent, and just 8.2 percent for carpenters, both trades which are considered to have a high percentage of illegal workers.

Donohue acknowledged "there's a lot of history'' of situations where an influx of immigrants into an area, presumably legal, have kept wages lower than they might be somewhere else.

"But it's very cyclic,'' Donohue continued. "When that happens, as soon as people get established, learn a skill, start performing, they're demanding the same thing that every new American wants is a little more of the piece of the apple,'' he said.

Donohue also said there is no evidence foreign born workers, no matter how they got here, are taking jobs from legal U.S. residents. As proof he cited a year-old study from the Pew Center for Hispanic Studies which showed no correlation between employment rates for native workers and the number of new immigrants in the workforce.

Donohue also chided those who believe that any federal immigration reform measure should include a requirement to send home those who already are working here even if they did enter the country illegally.

"It makes far greater sense to normalize the undocumented immigrants that are already here, that have a track record, that have a resume, instead of rounding them all up which you couldn't do anyway,'' he said.

"Why not take the workers that we've already got, why don't we make them some form of legal - I don't say you've got to make them citizens - and give them an opportunity to come out of the shadows?'' Donohue continued. He said forcing them to go home and then have employers look for workers amounts to "trading them in and starting all over again.''