PHOENIX - The Obama administration sued Arizona on Tuesday to throw out the state's toughest-in-the-nation immigration law and keep other states from copying it.

The lawsuit filed in federal court in Phoenix said the law, due to take effect July 29, usurps the federal government's "pre-eminent authority" under the Constitution to regulate immigration.

The move sets the stage for a high-stakes legal clash over states' rights at a time when politicians in some other states have indicated they want to follow Arizona's lead.

The legal action represents a stern denunciation of the law, which the Justice Department declared will "cause the detention and harassment of authorized visitors, immigrants and citizens who do not have or carry identification documents" while ignoring "humanitarian concerns" and harming diplomatic relations.

Supporters of the law said the lawsuit was unnecessary and blamed the federal government for neglecting problems at the border for years. Republican Gov. Jan Brewer called the complaint "a terribly bad decision" and defended the law as "reasonable and constitutional."

Arizona passed the measure after years of frustration with illegal immigration, including drug trafficking, kidnappings and murders. The state is the biggest gateway into the U.S. for illegal immigration, and it's home to an estimated 460,000 illegal immigrants.

The law requires police, while enforcing other laws, to question a person's immigration status if there's reasonable suspicion that the person is in the country illegally. It also requires legal immigrants to carry their immigration documents and bans day laborers and people who seek their services from blocking traffic on streets.

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 heart of the legal arguments focus on the Constitution's assertion that federal laws override state laws. The lawsuit says that comprehensive federal laws already on the books cover illegal immigration - and that those statutes take precedent.

"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 lawsuit also says that the Arizona measure will impose a huge burden on U.S. agencies in charge of enforcing immigration laws, "diverting resources and attention from the dangerous aliens who the federal government targets as its top enforcement priority."

The next step is for the case to be assigned a judge, who will decide whether to grant a preliminary injunction to temporarily block the law from taking effect.

Brewer predicted that the law would survive the federal challenge as well as pending suits previously filed by private groups and individuals.

"As a direct result of failed and inconsistent federal enforcement, Arizona is under attack from violent Mexican drug and immigrant smuggling cartels. Now, Arizona is under attack in federal court from President Obama and his Department of Justice," Brewer said. "Today's filing is nothing more than a massive waste of taxpayer funds."

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.

The lawsuit is sure to have legal and political ramifications beyond Arizona as the courts weigh in on balancing power between the states and the federal government and politicians invoke the immigration issue in a crucial election year.

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. On the Republican side, Sen. John McCain is locked into a tough primary fight as his right-leaning GOP challenger takes him to task for his earlier promotion of comprehensive immigration reform, which he has since abandoned in favor of a message to "complete the danged fence."

The case focuses heavily on the legal argument called pre-emption - an issue that has been around since the Founding Fathers declared that the laws of the United States "shall be the supreme law of the land."

The Obama administration's reliance on the pre-emption argument in the Arizona case marks the latest chapter in its use of this legal tool.

Within months of taking office, the Obama White House directed department heads to undertake pre-emption of state law only with full consideration of the legitimate prerogatives of the states.

The 2009 directive was aimed at reversing Bush administration policy that had aggressively employed pre-emption in an effort to undermine a wide range of state health, safety and environmental laws.

"The case strikes me as incredibly important because of its implications for the immigration debate," said University of Michigan constitutional law professor Julian Davis Mortenson. "The courts are going to take a close look at whether the Arizona law conflicts with congressional objectives at the federal level."

Kris Kobach, the University of Missouri-Kansas City law professor who helped draft the Arizona law, said he's not surprised by the Justice Department's challenge and called it "unnecessary."

He noted that the law already is being challenged by the American Civil Liberties Union and other groups opposed to the new statute.

"The issue was already teed up in the courts. There's no reason for the Justice Department to get involved. The Justice Department doesn't add anything by bringing their own lawsuit," Kobach said in an interview.

The Mexican government welcomed the move, saying the law "affects the civil and human rights of thousands of Mexicans."

Associated Press writers Paul Davenport and Jonathan J. Cooper in Phoenix, John Hanna in Topeka, Kan., and Pete Yost in Washington contributed to this report.