The federal judge overseeing former President Donald J. Trump’s classified documents case said on Thursday that she intended to look anew at a hugely consequential legal victory that prosecutors won last year and that served as a cornerstone of the obstruction charges filed against Mr. Trump.

In her ruling, the judge, Aileen M. Cannon, said she would hold a hearing to reconsider another judge’s decision to allow prosecutors to pierce the attorney-client privilege of one of Mr. Trump’s lawyers under what is known as the crime-fraud exception. ...

Depending on how Judge Cannon ultimately rules, her decision to redo the fraught and lengthy legal arguments about the crime-fraud exception could deal a serious blow to the obstruction charges in the indictment of Mr. Trump.