The federal government only occasionally intervenes forcefully in a state’s affairs, and it carries significant political risks. With immigration continuing to be a hot issue in political campaigns across the country, the Arizona law, which grants the local police greater authority to check the legal status of people they stop, has become a rallying cry for the Tea Party and other conservative groups.

The lawsuit, though widely anticipated, was confirmed by an unexpected source: Secretary of State Hillary Rodham Clinton, who answered a question about it from an Ecuadorean TV journalist in an interview on June 8 that went all but unnoticed until this week.

Noting that President Obama had publicly objected to the law, Mrs. Clinton said, “The Justice Department, under his direction, will be bringing a lawsuit against the act."

A spokesman for the Justice Department said the matter was still under review, but other senior administration officials, speaking on the condition of anonymity, said a decision had indeed been made and only the details of the legal filing were still being worked out.