WASHINGTON — The House is expected to vote Thursday on a Democratic resolution that will lay out the next steps in the impeachment inquiry, according to a senior congressional source.

The measure — whose language has not yet been released — is expected to detail procedures going forward in the investigation, not to formalize it.

“This resolution establishes the procedure for hearings that are open to the American people, authorizes the disclosure of deposition transcripts, outlines procedures to transfer evidence to the Judiciary Committee as it considers potential articles of impeachment, and sets forth due process rights for the President and his Counsel,” House Speaker Nancy Pelosi, D-Calif., said in a letter to her caucus Monday.

“We are taking this step to eliminate any doubt as to whether the Trump Administration may withhold documents, prevent witness testimony, disregard duly authorized subpoenas, or continue obstructing the House of Representatives.”

The move comes as pressure grows on Democrats to conduct the inquiry with a more open process, including holding public hearings with key witnesses in the Ukraine case. Republicans have been calling on House Democrats to hold a floor vote to formalize the impeachment inquiry so that Republicans can be given subpoena power, but Pelosi has decided against such a vote. Trump administration officials also have called for a House vote on the authorization of the inquiry, calling the probe invalid without one and refusing to cooperate with the investigation.

Pelosi said in her letter Monday that Democrats "will bring a resolution to the Floor that affirms the ongoing, existing investigation that is currently being conducted by our committees as part of this impeachment inquiry, including all requests for documents, subpoenas for records and testimony, and any other investigative steps previously taken or to be taken as part of this investigation.”

The House Rules Committee on Monday announced that its members will meet on Wednesday afternoon, in advance of the Thursday vote, to consider the resolution. A description of the proposal on the committee's website said the measure would direct "certain committees to continue their ongoing investigations as part of the existing House of Representatives inquiry into whether sufficient grounds exist for the House of Representatives to exercise its Constitutional power to impeach Donald John Trump, President of the United States of America.”

So far, nine witnesses have provided more than 70 hours of closed-door testimony, according to an tally by NBC News.

The Democrats' move comes after former deputy national security adviser Charles Kupperman refused to appear for a scheduled deposition Monday before the three House congressional committees involved in leading the impeachment inquiry.

The White House has been trying to block his appearance, and Kupperman, who worked under former national security adviser John Bolton, filed a lawsuit Friday asking a federal judge to rule on whether he must testify under a congressional subpoena.