WASHINGTON - President Obama once favored a "crackdown on employers" who hired illegal immigrants, and as a candidate called for "much tougher enforcement standards" for companies employing illegal workers.

But on Wednesday, Obama's top courtroom lawyer will join the U.S. Chamber of Commerce in urging the Supreme Court to strike down an Arizona law that goes after employers who hire illegal workers.

The administration also seeks to void a part of the state's law that tells employers they must check the federal government's E-Verify database to make sure their new hires are authorized to work in the United States.

The move sets the stage for a high court ruling on the most disputed issue in immigration law: Can states and cities enforce their own laws against illegal immigrants, or must they wait for federal authorities to act?

The administration found itself in an awkward spot in part because the Legal Arizona Workers Act was signed into law in 2007 by then-Gov. Janet Napolitano. She said it would impose the "business death penalty" on employers caught a second time hiring illegal workers, and blamed the "constant demand of some employers for cheap, undocumented labor" for the flow of illegal immigration into Arizona.

Now, however, Napolitano is Obama's secretary of Homeland Security, which enforces the immigration laws and administers E-Verify, a voluntary program that checks whether new hires are authorized to work in this country. Federal agencies and federal contractors are required to use it.

This year, the administration had a fierce internal debate over what to do about laws in Arizona and elsewhere against illegal immigration.

In November 2009, the high court asked the Justice Department to weigh in on the Legal Arizona Workers Act.

Several participants in the debate say Napolitano counseled against intervening in the case, Chamber of Commerce v. Whiting. She and others emphasized that the administration had tried to send the message that it favored strong enforcement.

But in the spring, immigrant-rights advocates stepped up the pressure and argued that the administration had to take a stand against a second Arizona law. S.B. 1070 required police to check the immigration status of people they have lawfully stopped and suspect are in the country illegally. Activists feared that law would lead to "racial profiling" and harassment of legal immigrants.

In May, shortly after Arizona Gov. Jan Brewer signed S.B. 1070, the Obama administration made its decision.

It urged the Supreme Court to hear the challenge to the Legal Arizona Workers Act on the grounds that it conflicted with the exclusive federal authority to enforce immigration laws.

In recent years, most states and many localities have considered laws that restrict or regulate illegal immigrants in areas such as employment, education or housing. Governors and lawmakers said they needed to act because the federal government had failed to enforce immigration laws.

Federal lawyers argued that these state and local laws conflicted with Washington's exclusive control over immigration enforcement.

In July, a federal judge in Phoenix, acting on a suit by the Obama administration, blocked S.B. 1070 from taking effect. That case is on appeal. For its part, the Supreme Court has not ruled squarely on the federal-vs.-state clash over immigration since 1976.