Economic research finds immigrants help fuel the nation's economic growth and have an overall positive effect on the income of native-born workers, according to a new White House study.

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Immigrant workers find employment that tends to complement -- not replace -- the jobs held by workers born in the United States, the paper said. On average, native-born U.S. workers' wages have been boosted as immigrant workers have helped to expand the nation's overall economic pie and thus its wealth by billions of dollars a year, the paper said.

Among the report's other findings:

• Immigrants make up 15 percent of all workers, and larger shares of occupations such as construction, food services and health care.
• About 40 percent of Ph.D. scientists working in the United States were born abroad.
• Immigrants and their children assimilate into U.S. culture. For example, although 72 percent of first-generation Latino immigrants use Spanish as their predominant language, only 7 percent of the second generation are Spanish-dominant.

The paper acknowledges the challenges of U.S.-born workers with little education, but adds that "it is safe to conclude that immigration is not a central cause of those difficulties, nor is reducing immigration a well-targeted way to help these low-wage natives."

The report came as a coalition of labor groups announced their vehement opposition to the immigration bill, denouncing it as "anti-worker."

"It creates a situation ripe for exploitation," said Richard L. Trumka, the AFL-CIO secretary-treasurer.

Immigrant-heavy service unions, on the other hand, are supporting the bill, which would swell their ranks.

White House Chides Compromises

The Bush administration came out strongly against a bipartisan effort by Sens. Charles Grassley and Barack Obama to make the immigration bill easier on employers.

Homeland Security Secretary Michael Chertoff told senators in a letter late Tuesday that the amendment, which makes a new program to stop businesses from hiring illegal workers less burdensome, "would be a serious step backwards in our enforcement effort."

The amendment sponsored by Grassley, R-Iowa, Max Baucus, D-Mont., and Obama, D-Ill. "eliminates needed tools and allows unscrupulous businesses to continue to freely hire illegal workers," Chertoff wrote in matching letters to Sens. Edward M. Kennedy, D-Mass., and Arlen Specter, R-Pa., two architects of the bill.

In an angrily worded reply to Chertoff on Wednesday, the unlikely allies sponsoring the amendment dismissed his criticism as "erroneous and misleading," and defended their proposal as one that would improve a deeply flawed system.

Their amendment is one of a limited list of two dozen the Senate would consider adding to the immigration measure under a plan to revive the stalled bill before the July 4 recess.

Consumed with a debate on energy policy, the Senate is unlikely to turn back to the bill until early next week. When it does, skeptical senators in both parties will get the lion's share of the opportunities to revise it through amendments that could cut to the heart of the measure.