Washington (CNN) Key House Democrats announced on Monday that they are issuing a subpoena to President Donald Trump's personal lawyer Rudy Giuliani for documents related to Ukraine as part of the House impeachment inquiry .

"Pursuant to the House of Representatives' impeachment inquiry, we are hereby transmitting a subpoena that compels you to produce the documents set forth in the accompanying schedule by October 15, 2019," wrote Rep. Adam Schiff, chairman of the House Intelligence committee, Rep. Eliot Engel, chairman of Foreign Affairs committee and Rep. Elijah Cummings, chair of the House Oversight committee in a letter to Giuliani.

The letter stated that the subpoena was being issued by the House Intelligence Committee and that the documents will be shared among the two other committees and the House Judiciary Committee.

The move is the latest escalation in the House impeachment inquiry that Democrats are pressing full steam ahead -- even during a two-week congressional recess -- after House Speaker Nancy Pelosi formally announced the inquiry in the wake of outrage over reports that the President had pressured a foreign leader in an effort to target a political rival.

A transcript of a July phone call between Trump and Ukrainian President Volodymyr Zelensky that was also released last week showed Trump repeatedly pushed for Zelensky to investigate former Vice President Joe Biden, and his son, Hunter. There is no evidence of wrongdoing by either Joe or Hunter Biden.

Giuliani was discussed as part of that phone call, according to the transcript , with Trump telling the Ukrainian President at one point, "I will have Mr. Giuliani give you a call."

In the letter, the committee chairmen reference an interview that Giuliani did with CNN's Chris Cuomo earlier this month, during which Giuliani denied asking Ukraine to investigate Biden moments before admitting that he had done just that.

JUST WATCHED Giuliani contradicts himself over Ukraine probe of Biden Replay More Videos ... MUST WATCH Giuliani contradicts himself over Ukraine probe of Biden 01:15

"In addition to this stark admission, you stated more recently that you are in possession of evidence -- in the form of text messages, phone records, and other communications -- indicating that you were not acting alone and that other Trump administration officials may have been involved in this scheme," the chairmen wrote.

They continued: "The subpoena requires you to produce all of those communications, and other related documents, to the committees in order to determine the full extent of this effort by the President and his administration to press Ukraine to interfere in our 2020 presidential election."

The chairmen warn Giuliani that if he does not comply with the subpoena, it will amount to obstruction.

"Your failure or refusal to comply with the subpoena, including at the direction or behest of the President or the White House, shall constitute evidence of obstruction of the House's impeachment inquiry and may be used as an adverse inference against you and the President," the chairmen wrote.

CNN has asked Giuliani for comment. He has previously denied any wrongdoing and defended his efforts to get a Ukrainian investigation launched.

In a tweet, Giuliani suggested he may not comply with the subpoena.

"I have received a subpoena signed only by Democrat Chairs who have prejudged this case," Giuliani said. "It raises significant issues concerning legitimacy and constitutional and legal issues including, inter alia, attorney client and other privileges. It will be given appropriate consideration."

In another sign that House Democrats are moving swiftly in their effort to investigate the President's contacts with Ukraine, the chairmen of the same three House committees who sent the letter to Giuliani on Monday notified Secretary of State Mike Pompeo at the end of last week that he is also being subpoenaed in connection with the Ukraine probe.

While Democrats only control the majority in the House and not in the Senate, Senate Democrats are also pushing to shape congressional oversight over the Ukraine matter.

Democratic senators on the Senate Judiciary Committee sent a letter to the committee's chairman Lindsey Graham, a South Carolina Republican, on Monday calling for hearings with Attorney General Bill Barr as well as other administration officials on the allegations raised in the whistleblower complaint.

Some Senate Democrats have also expressed concern over the safety of the whistleblower as the President goes on the attack against the individual.

The President has been on a tirade against the whistleblower on Twitter, tweeting on Monday morning, "#FakeWhistleblower."

In an interview with CNN, Democratic Sen. Mark Warner, who is vice chairman of the Senate Intelligence Committee, railed on the President's comments about the whistleblower, saying "any rational person would be concerned about the whistleblower's safety."

He would not say if Trump's comments are impacting his committee's ability to talk to the whistleblower, but said Trump's comments will have a "chilling effect" and impact others who want to come forward. He said his committee right now is deciding whom they want to interview, declining to comment on if they've issued subpoenas or if they want to speak with Giuliani, among others.

This story has been updated with additional developments on Monday.