A federal judge on Thursday rejected President George W. Bush’s contention that senior White House advisers are immune from subpoenas, siding with Congress’ power to investigate the executive branch and handing a victory to Democrats probing the dismissal of nine U.S. prosecutors.

The unprecedented ruling undercut three presidential confidants who have defied congressional subpoenas for information that Bush says is protected by executive privilege. Democrats swiftly announced they would schedule hearings in September, at the height of election season.