WASHINGTON  The Justice Department moved on Wednesday to drop all charges against former Senator Ted Stevens of Alaska, who lost his seat last year just days after being convicted on seven felony counts of ethics violations.

The case was one of the most high profile and bitterly fought in a string of corruption investigations into current and former members of Congress. But Justice Department lawyers told a federal court Wednesday that they had discovered a new instance of prosecutorial misconduct, on top of earlier disclosures that had raised questions about the way the case was handled, and asked that the convictions be voided.

The attorney general, Eric H. Holder Jr., said he would not seek a new trial.

Mr. Stevens, 85, was the longest-serving Republican in the history of the Senate and Alaska’s dominant political figure for more than four decades. His career mirrored the state’s rough-and-tumble journey from a remote territory to an economic powerhouse as he used his influence to send billions of dollars in federal aid to Alaska.

The collapse of the Stevens case was a profound embarrassment for the Justice Department, and it raised troubling issues about the integrity of the actions of prosecutors who wield enormous power over people they investigate. Mr. Stevens’s case was handled by senior officials of the department’s Public Integrity Section, which handles official corruption cases.