WASHINGTON (Reuters) - A federal judge on Thursday rejected a request by Republican U.S. Sen. Ted Stevens of Alaska to dismiss the charges or declare a mistrial.

His lawyers made the request because they said prosecutors withheld evidence helpful to their defense in the case of the powerful, veteran lawmaker who is accused of filing false financial disclosure forms by failing to include more than $250,000 in gifts he received.

U.S. Justice Department prosecutors admitted a serious mistake had been made, but had urged the judge to allow the trial to go forward. They said the oversight was not serious enough to warrant dismissing the case or ordering a new trial.

The judge sided with the prosecutors in rejecting the defense request, but said he had lost confidence in the ability of the prosecutors to turn over helpful evidence to the defense.

The judge’s decision to allow the trial to go forward was a setback for Stevens, 84, one of the Senate’s most powerful Republicans who is seeking reelection in November.

Stevens, who has been in the Senate for 40 years and is the longest-serving Republican senator in U.S. history, faces a close race in what has long been a safe Republican seat. His Democratic opponent is Anchorage Mayor Mark Begich.

U.S. District Judge Emmet Sullivan said he would adjourn the trial until Monday to give defense lawyers time to review new information that he ordered the prosecutors to turn over.