PHOENIX (AP) -- The U.S. Justice Department on Tuesday filed a lawsuit challenging the constitutionality of Arizona's new law targeting illegal immigrants, setting the stage for a clash between the federal government and the state over the nation's toughest immigration crackdown.
The lawsuit filed in U.S. District Court in Phoenix argues that Arizona's law requiring state and local police to question and possibly arrest illegal immigrants during the enforcement of other laws such as traffic violations usurps federal authority.
"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 declared invalid.
The government contends that the Arizona law violates the supremacy clause of the Constitution, a legal theory that says federal laws override state laws. It is already illegal under federal law to be in the country illegally, but Arizona is the first state to make it a state crime and add its own punishment and enforcement tactics.
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.
"It's outrageous and it's clear they don't want (immigration) laws enforced. What they want is to continue their non-enforcement policy," Pearce said. "They ignore the damage to America, the cost to our citizens, the deaths" tied to border-related violence.
State Rep. Kyrsten Sinema, a Phoenix Democrat who opposes the law, said the suit should help settle questions over what states can do when they don't think federal laws are being adequately enforced.
"I hope this galvanizes Congress to gain the moral courage they need to address this (immigration) crisis," Sinema said.
Tuesday's action has been expected for weeks. President Barack Obama has called the state law misguided. Supporters say it is a reasonable reaction to federal inaction on immigration.
Gov. Jan Brewer's spokesman called the decision to sue "a terribly bad decision."
"Arizona obviously has a terrible border security crisis that needs to be addressed, so Gov. Brewer has repeatedly said she would have preferred the resources and attention of the federal government would be focused on that crisis rather than this," spokesman Paul Senseman said.
Three of the five Democrats in Arizona's congressional delegation, who are facing tough re-election battles, had also urged Obama not to try to block the law from going into effect.
Republican Sens. Jon Kyl and John McCain of Arizona also lashed out at the administration's decision, saying "the American people must wonder whether the Obama Administration is really committed to securing the border when it sues a state that is simply trying to protect its people by enforcing immigration law."
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.
Arizona passed the law after years of frustration over problems associated with illegal immigration, including drug trafficking and violent kidnappings. The state is the biggest gateway into the U.S. for illegal immigrants, and is home to an estimated 460,000 illegal immigrants.
Obama addressed the Arizona law in a speech on immigration reform last week. He touched on one of the major concerns of federal officials, that other states were poised to follow Arizona by crafting their own immigration enforcement laws.
"As other states and localities go their own ways, we face the prospect that different rules for immigration will apply in different parts of the country," Obama said. "A patchwork of local immigration rules where we all know one clear national standard is needed."
The law makes it a state crime for legal immigrants to not carry their immigration documents and bans day laborers and people who seek their services from blocking traffic on streets.
The law also prohibits government agencies from having policies that restrict the enforcement of federal immigration law and lets Arizonans file lawsuits against agencies that hinder immigration enforcement.
Arizona State University constitutional law professor Paul Bender said the federal government's involvement throws a lot of weight behind the argument that federal law pre-empts Arizona's measure.
"It's important to have the federal government's view of whether state law is inconsistent with federal law, and they're the best people to say that," Bender said.
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 but called it "unprecedented and 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.
Associated Press Writers Paul Davenport and Jonathan J. Cooper in Phoenix and John Hanna in Topeka, Kan. contributed to this report.