Legal action has been widely expected, given Mr. Obama’s repeated statements against it, as well as the concerns that Attorney General Eric H. Holder Jr. has voiced in interviews and news conferences.

In late May, Justice Department lawyers traveled to Phoenix to speak with lawyers from the offices of the state attorney general, Terry Goddard, and Gov. Jan Brewer about the possibility of litigation. Mr. Goddard, who is seeking the Democratic nomination for governor, and Ms. Brewer, a Republican who is running for the office for the first time (Ms. Brewer succeeded former Gov. Janet Napolitano when Ms. Napolitano joined the Obama administration), both say a federal lawsuit is unwarranted.

In a side drama, Mr. Goddard on Friday took his office off the case, bowing to the wishes of Ms. Brewer, who had said his opposition to the law would make it difficult for him to defend it. Mr. Goddard said his decision had nothing to do with the Justice Department’s plans.

Mrs. Clinton’s disclosure — which came to light after her interview was posted by a political blog, therightscoop.com — quickly became fodder for political campaigns in Arizona. Republicans, led by Ms. Brewer, seized on the notion of a domestic policy decision’s being disclosed on foreign soil.

“This is no way to treat the people of Arizona,” the governor said in a statement. “To learn of this lawsuit through an Ecuadorean interview with the secretary of state is just outrageous. If our own government intends to sue our state to prevent illegal immigration enforcement, the least it can do is inform us before it informs the citizens of another nation.”

The federal government from time to time has successfully brought claims against laws it deemed discriminatory or infringing on voter rights. It also has a history of suing states on issues related to prison conditions and school desegregation, said Erwin Chemerinsky, a constitutional scholar at the law school at the University of California, Irvine.

While Arizona’s law has drawn opposition from those who worry that Hispanic-Americans and legal residents will be mistaken for illegal immigrants, legal scholars say the case will more likely to turn on whether it intrudes on federal immigration authority.