Despite the unveiling of substantial evidence against former President Donald J. Trump in the documents investigation, prosecutors suffered a potential setback with the surprising assignment of the case to Judge Aileen M. Cannon. A Trump appointee, Judge Cannon had intervened in the investigation and issued rulings favorable to Mr. Trump last year, only to be rebuked by a conservative appeals court. It is unclear whether Mr. Trump’s case was randomly delegated to Judge Cannon or if an exception was made, but legal specialists emphasized the significance of Judge Cannon’s return. As the case involves classified evidence, any judge would likely spend significant time in handling pretrial hearings over whether to allow substitutions that do not contain classified information. The decisions Judge Cannon makes in establishing the pretrial and trial calendar could also impact the case’s tempo and outcome. Mr. Trump and his legal team have already signaled their intent to raise accusations of misconduct by prosecutors and investigators, and if Judge Cannon entertains those claims more deferentially than trial judges normally do, Mr. Smith and his team could effectively go on trial before Mr. Trump.