House Democrats have called on White House Chief of Staff Mick Mulvaney to testify as part of their partisan impeachment investigation into President Donald Trump.

“Based on evidence gathered in the impeachment inquiry and public reporting, we believe that you possess substantial first-hand knowledge and information relevant to the House’s impeachment inquiry,” House Intelligence Chairman Adam Schiff (D-CA), House Foreign Affairs Chairman Eliot Engel (D-NY), and acting House Oversight Chairwoman Carolyn Maloney (D-NY) stated in a letter to Mulvaney on Tuesday.

JUST IN: House Democrats request acting White House chief of staff Mick Mulvaney give deposition in impeachment probe. https://t.co/F2Y3hyQ58Y pic.twitter.com/t7tIn8WA4a — ABC News (@ABC) November 5, 2019

The development comes as House lawmakers leading the impeachment investigation will release more deposition transcripts, which will detail testimony from an administration diplomat and a former U.S. envoy to Ukraine.

The chairs of the House intelligence, foreign affairs, and oversight committees will publicize the transcripts from U.S. Ambassador to the European Union Gordon Sondland and former U.S. special representative to Ukraine Kurt Volker. Both were deposed last month by committee investigators — Sondland after he was initially blocked by the Trump administration from providing testimony.

Sondland testified on Oct. 17 and Volker on Oct. 3.

Both are considered important witnesses in the investigation, which seeks to determine whether President Trump threatened to withhold military aid to Kiev to pressure Ukrainian officials to investigate Democratic presidential candidate Joe Biden and his son, Hunter, a former Ukrainian gas company executive.

Both President Trump and Ukrainian President Volodymyr Zelensky have denied any pressure was applied to probe into the Biden family’s allegations of corruption and the White House released a transcript of the world leaders’ phone call in an attempt to show no wrongdoing occurred.

“There was no blackmail,” Zelensky told reporters of July 25th call last month. “We are not servants. We are an independent country.”

The UPI contributed to this report.