During the first two years of the Trump administration, when Democrats were in the minority, they bristled when officials like Mr. Sessions refused to answer their questions about communications with the president on the grounds that Mr. Trump might, in the future, want to invoke executive privilege over them — even though Mr. Trump never actually did so.

Executive privilege is a power that presidents have claimed under the Constitution to prevent Congress from gaining access to internal executive branch information, like confidential communications between the president and his advisers. But such information is not legally shielded from Congress by default; rather, the president is supposed to choose whether to invoke executive privilege in any particular instance — at the cost of accepting any political fire for keeping that information secret.

With Democrats now in control of the House, Mr. Nadler is trying to avoid such stonewalling.

On Jan. 22, Mr. Nadler sent Mr. Whitaker a letter outlining questions that Democrats might ask him about his conversations with Mr. Trump and asked him to consult the White House ahead of time about whether Mr. Trump would invoke the privilege over them. Absent such an invocation, Mr. Nadler wrote that the committee would otherwise expect answers at the hearing.

If a witness refuses to answer a question during a congressional hearing without a legal right to do so, the House can vote to hold him in contempt. Congress can then ask the Justice Department to prosecute him or file a lawsuit asking a judge to order the witness to provide the information, raising the further possibility of being imprisoned for contempt of court. But a valid assertion of executive privilege would provide a lawful basis for declining to answer.

In 2012, Eric H. Holder Jr. was the first attorney general to be held in contempt of Congress for refusing to comply with a subpoena for information. Justice Department officials then declined to prosecute Mr. Holder, saying they had the discretion to not prosecute an executive branch official who claims executive privilege.

But the Justice Department has long considered a subpoena to be a necessary precursor to Congress holding an executive branch official in contempt. Its letter to Mr. Nadler demanding that he agree not to issue the subpoena during the hearing, therefore, amounted to a demand that he not initiate contempt proceedings against Mr. Whitaker during the hearing and instead commit to negotiating over any disputed information afterward, once any questions have actually been asked.

“We cannot understand this measure other than as an attempt to circumvent the constitutionally required accommodation process and thereby to transform the hearing into a public spectacle,” the department’s letter said.