The US House of Representatives plans to take its first formal vote this week on the impeachment inquiry into President Donald Trump.
House Speaker Nancy Pelosi said the vote "sets forth due process rights for the President and his Counsel".
The president and his allies have argued the lack of a vote by the full chamber renders the inquiry invalid.
The House is investigating claims Mr Trump pressured a foreign government to investigate his political rivals.
Thursday's vote planned by Mrs Pelosi and her fellow Democrats is not to impeach Mr Trump, but to set out ground rules for their inquiry.
Mrs Pelosi, the most powerful elected Democrat, has until now rebuffed calls from Republicans to hold any such formal vote.
But in her letter to fellow Democrats on Monday, the California congresswoman pointed out the US constitution does not require such a step.
She said the move would "eliminate any doubt" as to whether the White House can withhold documents, disregard subpoenas or prevent witnesses from giving testimony. Several administration officials have failed to testify to committees involved in the inquiry.