Q+A-Arizona immigration law expected to be challenged

PHOENIX, June 30 (Reuters) - The Obama administration is expected to file a lawsuit challenging Arizona's strict new law that aims to crack down on illegal immigration.

Here are some questions and answers about the issue and what could happen.

WHAT IS THE ARIZONA IMMIGRATION LAW?

The law requires state and local police to investigate the immigration status of anyone they reasonably suspect is an illegal immigrant. It also makes it a state crime to transport or harbor unauthorized immigrants, and to hire day laborers off the street. It takes effect on July 29 unless challenged.

WHY IS IT CONTROVERSIAL?

Immigration issues have usually been dealt with at a federal level, rather than by states. President Barack Obama has spoken out against the precedent set by the Arizona immigration law, warning that it could lead to a 'patchwork' of conflicting state laws across the country.

Opponents of the law argue that it is unconstitutional and a mandate for racial profiling, and fear it will destroy trust between Hispanic communities and law enforcement in the Mexico border state. Opponents have so far lodged five separate challenges in federal court seeking to derail the law.

WHY DID ARIZONA PASS IT?

The desert border state straddles the most heavily trafficked corridor into the United States for human and drug smugglers from Mexico. The U.S. government estimated that 460,000 unauthorized immigrants lived in the border state in January 2009. So far this year, U.S. Border Patrol agents have made an average of around 700 arrests a day.

Republicans who control the state legislature charge that the federal government has failed in its duty to secure the border. They hope the law will drive unauthorized immigrants out of the state and help to curb border-related crime.

Following the passage of the law, Obama pledged 1,200 National Guard troops to boost security along the southwest border. In late June, the administration announced that it would deploy 524 of those troops to the Arizona-Mexico border by August.

WHAT RISKS DOES IT POSE IN THIS ELECTION YEAR?

Polls consistently show that the state law is supported by a solid majority of Americans, although it carries considerable political risks for both parties.

Republicans have insisted that the border be secured first. However, by embracing it, Republicans could lose more ground with Hispanics, the biggest and fastest growing minority in the United States, creating long-term political difficulties.

But dozens of endangered Democrats in conservative districts, already at risk after casting tough votes for healthcare reform and more spending, would be even bigger targets for grassroots conservative activists.

HOW HAS IT IMPACTED COMPREHENSIVE IMMIGRATION REFORM?

The furor over the law immediately jolted Democrats efforts to push comprehensive immigration reform. Senate Majority Leader Harry Reid, a Nevada Democrat, unveiled a framework for overhauling the country's immigration system days after the Arizona law was passed. It has not won Republican support needed to pass a bill, and analysts say prospects for success are slim before the November congressional elections.

Obama promised to make overhauling immigration laws a priority during his first year in office. He supports a system that allows undocumented immigrants in good standing to pay a fine, learn English and become citizens. He also backs tightening border security and clamping down on employers that hire undocumented workers.

Obama met with Hispanic and immigration activists at the White House in late June and is slated to give a speech on the need for comprehensive immigration reform on Thursday.

WHAT ABOUT A LEGAL CHALLENGE?

The U.S. Justice Department is expected to file a lawsuit challenging the Arizona law, a move that could coincide with Obama's speech on Thursday.

U.S. Attorney General Eric Holder, the top law enforcement official in the country, has described the Arizona law as an "unfortunate one", subject to potential abuse and that it could create a wedge between communities and law enforcement.

If the administration goes ahead with the lawsuit, the Justice Department could ask the court for a stay that would prevent it from taking effect until the legal process has concluded -- which could take years if challenges go all the way to the U.S. Supreme Court.

Winning a stay could be difficult, however, because the threshold is high. The Justice Department would have to prove that the implementation of the law would cause irreparable harm.

Nonetheless, some experts believe a stay is possible and that the law may undermine federal powers on dealing with immigration status, making it vulnerable in a court challenge. (Additional reporting by Jeremy Pelofsky in Washington)