Kenneth Chesebro, an architect of the plan to deploy people claiming to be Trump electors in states won by President Biden, is cooperating with inquiries in Michigan, Arizona and Nevada.
As the prosecutor, Fani T. Willis, discussed the case at a conference, her office sought an emergency protective order to prevent more leaks of discovery materials.
The former president is facing down Michael Cohen, his longtime fixer, in a Manhattan courtroom, while other ex-loyalists are cooperating in a case against him in Georgia.
The guilty pleas present a host of challenges as state and federal prosecutors engage in parallel efforts to hold Donald J. Trump accountable, defense lawyers and former federal prosecutors say.