After months of criticism from Republicans who said President Obama was relaxing immigration enforcement in workplaces, the scope of the administration?s strategy has become clear as long-running investigations of employers have culminated in indictments, convictions, exponentially increased fines and jail sentences. While conducting fewer headline-making factory raids, the immigration authorities have greatly expanded the number of businesses facing scrutiny and the cases where employers face severe sanctions.

In a break with Bush-era policies, the number of criminal cases against unauthorized immigrant workers has dropped sharply over the last two years.

Among the employers who have felt the impact of the administration?s tactics are two owners of Mexican restaurants in the Chuy?s Mesquite Broiler chain, which are popular for their laid-back Margaritaville mood and their broiled mahi tacos. On April 20, immigration agents descended on 14 Chuy?s restaurants in coordinated raids in Arizona and California, detaining kitchen workers and carrying away boxes of payroll books and other evidence.

But at the arraignment days later in federal court here, no immigrant workers stood before the judge. The only criminal defendants were the owners, Mark Evenson and his son Christopher, and an accountant who worked with them, Diane Ingrid Strehlow. If the Evensons are convicted on all charges against them of tax fraud and harboring illegal workers, they each could face more than 80 years in jail.

Of 42 illegal immigrants caught in the Chuy?s sweep, only one was charged with a crime, and it was not related to the raid. Thirteen workers were processed for immigration violations ? which are civil offenses ? and detained or deported. The others remained in this country as witnesses or to seek legal status through the immigration courts.

Under President George W. Bush, immigration agents frequently conducted high-profile factory raids, leading away scores of unauthorized workers in handcuffs, often to face jail time for document fraud or identity theft before being deported. After a raid in Postville, Iowa, in 2008, nearly 300 immigrant workers went to federal prison.

The Chuy?s prosecution contrasted with the application by state and county authorities of a law that Arizona adopted in 2007 to punish employers who hire illegal immigrants; the measure was upheld by the Supreme Court on Thursday. Despite the political furor over that law, only a handful of cases have been brought against employers under its terms, which provide mainly for civil penalties. But state authorities have continued to bring criminal cases against illegal immigrant workers, leading to their deportations.

The Obama administration?s record on workplace enforcement has been fiercely debated in Washington since President Obama announced that he would try, against steep odds, to pass an immigration overhaul this year. Administration officials say that their audits and investigations of employers have laid the groundwork for a system that would dissuade companies from hiring illegal immigrants.

?We have steadily increased our efforts to investigate and prosecute employers who violate the law on a serious and grand scale,? said John Morton, the director of Immigration and Customs Enforcement, also known as ICE. The next step, administration officials said, is to open a pathway that would allow millions of illegal immigrants in the country to live and work here legally.

Republicans, pointing to the decline in arrests of unauthorized workers, say the administration is failing to remove those immigrants from the work force just when Americans are grappling with high rates of unemployment.