Democrats want to know how a Trump megadonor became postmaster general.

Senator Chuck Schumer of New York, the minority leader, called on the Postal Service board of governors on Wednesday to release information about the selection process that led to Louis DeJoy becoming postmaster general in May, charging that the board had denied access to the information.

The agency has blocked lawmakers from questioning Russell Reynolds, the firm involved in selecting Mr. DeJoy, by refusing to release the firm from a nondisclosure agreement, Mr. Schumer said.

Mr. Schumer said his office had sought a briefing from Kimberly Archer, a leader of the firm’s global nonprofit practice, so “Congress could satisfy its oversight obligations to better understand the selection of Mr. DeJoy,” who is a former C.E.O. of a transportation-logistics company that does business with the Postal Service.

In July, the board had deemed much of the information sought by lawmakers to be confidential, Mr. Schumer said.