PHOENIX - The federal government took a momentous step into the immigration debate Tuesday when it filed a lawsuit seeking to throw out Arizona's crackdown on illegal immigrants, calling it a law that blatantly violates the Constitution.

The lawsuit filed in federal court in Phoenix sets the stage for a high-stakes legal clash over states' rights at a time when politicians across the country have indicated they want to follow Arizona's lead on the toughest-in-the-nation immigration law.

The suit declares that the law will "cause the detention and harassment of authorized visitors, immigrants and citizens who do not have or carry identification documents" while altogether ignoring "humanitarian concerns" and harming diplomatic relations.

Supporters of the law say the suit was an unnecessary action by the federal government after years of neglecting problems at the border. Republican Gov. Jan Brewer called the lawsuit "a terribly bad decision." Arizona passed the law after years of frustration over problems associated with illegal immigration, including drug trafficking, kidnappings and murders.

The law requires officers, while enforcing other laws, to question a person's immigration status if there's a reasonable suspicion that they are in the country illegally. The law also makes it a state crime for legal immigrants to not carry their immigration documents.

Other states have said they want to take similar action - a scenario the government cited as a reason for bringing the lawsuit.

"The Constitution and the federal immigration laws do not permit the development of a patchwork of state and local immigration policies throughout the country," the suit says.

The legal arguments focus on the Supremacy Clause of the Constitution, which says federal laws override state laws.

"In our constitutional system, the federal government has pre-eminent authority to regulate immigration matters," the lawsuit says. "This authority derives from the United States Constitution and numerous acts of Congress. The nation's immigration laws reflect a careful and considered balance of national law enforcement, foreign relations, and humanitarian interests."

The government is seeking an injunction to delay the July 29 implementation of the law until the case is resolved. It ultimately wants the law struck down.

State Sen. Russell Pearce, the principal sponsor of the bill co-sponsored by dozens of fellow Republican legislators, denounced the lawsuit as "absolute insult to the rule of law," as well as to Arizona and its residents.

Reflecting the political delicacy of the issue, three Democratic members of Congress in Arizona asked the Obama administration not to bring the suit in a year when they face tough re-election battles.

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