House Democrats say they plan to subpoena the White House on Friday for a host of documents as part of the House's impeachment inquiry into President Donald Trump and Ukraine.

The subpoena threat to the White House escalates the impeachment investigation from House Democrats fueled by the US intelligence community's inspector general's review of a whistleblower complaint alleging President Donald Trump sought help from Ukraine's government to interfere in the 2020 elections. Wednesday's announcement follows subpoenas to Secretary of State Mike Pompeo and Trump's personal attorney Rudy Giuliani.

House Oversight Chairman Elijah Cummings, who is issuing the subpoena as part of the probe along with the Intelligence and Foreign Affairs Committees, sent a memo to members saying the subpoena was being issued after the White House would not provide documents.

"The White House's flagrant disregard of multiple voluntary requests for documents—combined with stark and urgent warnings from the Inspector General about the gravity of these allegations—have left us with no choice but to issue this subpoena," the memo reads.

The subpoena includes requests for documents related to Trump's July 25 call with Ukrainian President Volodymyr Zelensky, as well as the holding up of Ukrainian foreign aid.

The memo, which noticed Cummings planned to issue a subpoena Friday, comes as several House committees are bracing for a last-minute briefing from the State Department's Inspector General.

Cummings said that the "committees are investigating the extent to which President Trump jeopardized national security by pressing Ukraine to interfere with our 2020 election and by withholding security assistance provided by Congress to help Ukraine counter Russian aggression, as well as any efforts to cover up these matters."

Schiff told reporters Wednesday that obstruction of justice could be an article of impeachment against Trump, and efforts to block information from the committees could be viewed as corroborating the allegations, saying Democrats are "deeply concerned about Secretary Pompeo's effort now to potentially interfere with witnesses whose testimony is needed."

"Any effort by the Secretary, by the President, or anyone else, to interfere with the Congress' ability to call before it relevant witnesses will be considered as evidence of obstruction of the lawful functions of Congress," Schiff said during House Speaker Nancy Pelosi's weekly news conference, adding "more than that it will allow an adverse inference to be drawn as to the underlying facts.

Schiff added that Democrats "don't want this to drag on for months and months."

Schiff's comments come hours before State Department inspector general Steve Linick plans to "provide staff with copies of documents related to the State Department and Ukraine" Wednesday afternoon in a bipartisan closed-door briefing with relevant congressional committees.

When asked how Democrats can work with the President amid the impeachment inquiry, Pelosi responded that "they have nothing to do with each other," adding that while Democrats should uphold the Constitution, "we also have a responsibility to get the job done for the American people."

Pelosi also had a message for her caucus urging Democrats to "be fair to the President" and that the party has "to be worthy of the Constitution as we move forward."

She added, "I say to my colleagues: calmness, quiet, so that we can hear what is being said in this regard."

Trump appeared to be watching Pelosi's news conference, tweeting responses to Pelosi's claim that Democrats would continue to work on lowering drug prices, a trade deal and gun control legislation.

"Nancy Pelosi just said that she is interested in lowering prescription drug prices & working on the desperately needed USMCA," Trump tweeted. "She is incapable of working on either. It is just camouflage for trying to win an election through impeachment. The Do Nothing Democrats are stuck in mud!"

This story has been updated with additional developments Wednesday.