WASHINGTON – Lawmakers grappled yesterday with the impact of immigration on American employment at a time when President Bush favors a guest worker program that would match willing employers with employees from other countries.

Members of the Republican-dominated House Subcommittee on Immigration, Border Security, and Claims hotly debated two recent reports suggesting that all new jobs created between 2000 and 2004 were taken by new immigrants, while native-born Americans remained unemployed.

Economists testifying before the committee also clashed over whether immigration will ease or worsen the financial strains on the Social Security system.

A recent study by the Center for Immigration Studies, a Washington think tank that favors lower immigration levels, found that between March 2000 and March 2004 the number of unemployed adult Americans increased by 2.3 million, while the number of employed adult immigrants increased by the same amount.

While not every immigrant takes a job that was lost by a native-born American, the dramatic increase in the number of immigrants holding jobs certainly “calls into question the wisdom of proposals to increase immigration levels,” the institute’s director of studies, Steven Camarota, concluded.

A separate study from the Center for Labor Market Studies at Northeastern University in Boston reached similar conclusions, finding that “for the first time in the post-WWII era, new immigrations accounted for all the growth in employment over a four-year period.”

Both reports used Census Bureau data.

The chairman of the subcommittee, Rep. John Hostettler, a Republican of Indiana, called the results of two studies “astounding.”

“For struggling American workers, current immigration levels can prove challenging during good times. In bad times, they can be devastating,” he said.

But some Republicans and Democrats questioned the findings, suggesting that the relationship between employment and immigration was more complicated than opponents of immigration suggested.

Rep. Loretta Sanchez, a Democrat of Southern California, said low-skill workers are having trouble finding jobs due to competition from companies that have moved operations overseas, and because of a low minimum wage that doesn’t meet the needs of native-born Americans.

Competition with immigrants is “small potatoes” for low-skilled and low-income workers compared to other barriers they face, including advances in technology, poor education and housing, lack of child care, lack of financing and skills, and racial discrimination, an associate dean of public policy at Georgetown University, Harry Holzer, who called the studies’ findings “superficial.”

Immigration “cannot possibly account for” the job losses, he said, because new immigrant jobs have been concentrated in a small number of sectors such as maintenance, food preparation, and construction, while native job losses occurred elsewhere. For example, the number of manufacturing jobs declined by about 3 million over the four years, but new immigrants took only 335,000 jobs in the sector, he said.

Witnesses and lawmakers also clashed over whether low-wage immigrants are taking jobs that Americans do not want, or whether they are “displacing” low-skill native-born Americans by accepting lower wages.

Rep. Maxine Waters, a Democrat of California, said there are some jobs Americans are unwilling to take.

“You are not going to find people flowing to agriculture to pick grapes or lettuce. Everybody knows that,” she said.

Ms. Waters proposed making employers criminally liable for hiring undocumented immigrants and not paying them benefits, and she urged an increase in the minimum wage.

Rep. Daniel Lungren, a Republican of Sacramento, questioned Mr. Camarota’s assertion that unemployed Americans are willing to take any available job.

Under questioning, Mr. Camarota conceded that if immigration were completely cut off, many jobs in the agricultural sector would be replaced by “mechanization” rather than filled by native-born American workers.

“We are not going to have those jobs taken by Americans. We’re just going to eliminate them,” Mr. Lungren remarked.

On the issue of Social Security, Mr. Holzer argued that low birth rates in America mean that immigrants will eventually account for all the growth in the labor force, and “we need the workers to pay the taxes to pay for the benefits we all expect,” he said.

But Mr. Camarota countered that since many immigrants earn lower-than-average wages, they are likely to collect more money from the redistributive entitlement program than they pay in. “Overall, immigration is problematic for the Social Security system,” he said.