The lawsuit, which three sources said could be filed as early as Tuesday, will invoke for its main argument the legal doctrine of "preemption," which is based on the Constitution's supremacy clause and says that federal law trumps state statutes. Justice Department officials believe that enforcing immigration laws is a federal responsibility, the sources said.

A federal lawsuit will dramatically escalate the legal and political battle over the Arizona law, which gives police the power to question anyone if they have a "reasonable suspicion" that the person is an illegal immigrant. The measure has drawn words of condemnation from President Obama and Attorney General Eric H. Holder Jr. and opposition from civil rights groups. It also has prompted at least five other lawsuits. Arizona officials have urged the Obama administration not to sue.

Secretary of State Hillary Rodham Clinton first revealed last month that the Justice Department intended to sue Arizona, and department lawyers have been preparing their case, said the sources, who spoke on condition of anonymity because the government has not announced its plans. The filing is expected to include declarations from other U.S. agencies saying that the Arizona law would place a undue burden on their ability to enforce immigration laws nationwide, because Arizona police are expected to refer so many illegal immigrants to federal authorities.

The preemption doctrine has been established in Supreme Court decisions, and some legal experts have said such a federal argument likely would persuade a judge to declare the law unconstitutional.

But lawyers who helped draft the Arizona legislation have expressed doubt that a preemption argument would prevail. The law, signed by Gov. Jan Brewer (R) in April, is scheduled to take effect later this month.