The committees are investigating whether Trump compromised U.S. national security when he asked Zelensky for a “favor,” as described in a White House readout of a July 25 phone call between the two presidents, involving an investigation of Biden. Top Democrats have said Trump violated his oath of office and abused his power to extort a foreign leader to interfere in the 2020 presidential election on his behalf, and the allegations recently convinced Speaker Nancy Pelosi to formalize an impeachment inquiry.

Schiff, Engel and Cummings cited Giuliani’s recent acknowledgment that he asked the Ukrainian government to probe Biden and his son Hunter, demanding that Giuliani produce “text messages, phone records, and other communications” that could reveal the involvement of senior Trump administration officials.

The three committees and their chairmen have been thrust to the forefront of the House’s investigation into a whistleblower complaint centering on Trump’s phone call with Zelensky. The issue has united the House Democratic Caucus in outrage, prompting nearly every Democrat to support the resulting impeachment inquiry.

The complaint, which was initially withheld from Congress at the Justice Department’s behest, alleges that White House officials sought to “lock down” information relating to Trump’s phone calls with foreign leaders on a highly classified server. It also states that officials were “deeply disturbed” about Trump’s conduct, which they believed could constitute evidence that he abused his power and his office.

The intelligence community’s inspector general, Michael Atkinson, deemed the complaint “urgent” and “credible.” Atkinson will brief members of the Intelligence Committee behind closed doors on Friday.

Lawmakers are also examining Trump’s decision to freeze critical military aid to Ukraine around the same time as his phone call with Zelensky. Last week, the chairs of the House Budget and Appropriations committees demanded information from the White House as it seeks to establish a timeline for “when, why, and how the president and [the Office of Management and Budget] withheld this funding.”

Giuliani’s refusal to comply with the subpoena would “constitute evidence of obstruction of the House’s impeachment inquiry and may be used as an adverse inference against you and the president,” the three committee chairmen wrote in a letter to the former New York City mayor.

House Democrats have narrowed the focus of their impeachment inquiry to the Ukraine scandal and the White House’s longstanding resistance to congressional investigations.

The three committees are demanding that Giuliani turn over the information by Oct. 15. The panels also scheduled depositions with three of Giuliani’s business associates: Lev Parnas, Igor Fruman and Semyon “Sam” Kislin.

Giuliani, who has appeared on national television in recent days to defend himself and Trump, has represented the president throughout former special counsel Robert Mueller’s investigation and its aftermath.

A White House spokesman declined to comment on the subpoena to Giuliani.

The committees have been looking into the Ukraine allegations since June, but their probe has intensified after Giuliani suggested that he was in contact with senior Trump administration officials about an effort to probe Biden’s role in the removal of a Ukrainian prosecutor widely viewed as corrupt.

Former U.S. Ambassador to Ukraine Marie Yovanovitch, who came under fire from Giuliani and later returned to Washington, will sit for a deposition before the committees on Wednesday. Kurt Volker, the former U.S. special representative for Ukraine negotiations, is scheduled to be deposed on Thursday. Volker, who resigned last week, is said to have facilitated some of Giuliani’s interactions with Ukrainian officials.

The Foreign Affairs Committee subpoenaed Secretary of State Mike Pompeo last week for documents related to Trump’s July 25 phone call with Zelensky and other records related to Giuliani’s involvement. Those records are due by Friday.

In a statement to POLITICO, Giuliani said: “I have received a Committee subpoena from three Committees of the House. It raises substantial constitutional and legal issues as well as attorney-client and other privileges. These and other issues must all be considered before a proper decision can be made.”

Trump has maintained that he did nothing wrong in his interactions with Zelensky. He and his allies have gone after Schiff in recent days as the longtime Trump antagonist takes on a prominent role in the House’s impeachment inquiry.

On Monday, Trump said Schiff should be arrested for treason.

“Rep. Adam Schiff illegally made up a FAKE & terrible statement, pretended it to be mine as the most important part of my call to the Ukrainian President, and read it aloud to Congress and the American people,” Trump wrote on Twitter, referring to Schiff’s exaggerated paraphrase of the president’s phone call with Zelensky.

Trump told reporters later Monday that “we’re trying to find out” the identity of the whistleblower, even though he or she is entitled to anonymity under federal law. That suggestion has opened Trump up to allegations of witness intimidation.

Senate Democrats on Monday called on Senate Judiciary Committee Chairman Lindsey Graham (R-S.C.) to hold hearings with Attorney General William Barr and White House Counsel Pat Cipollone, among others, to more fully investigate the whistleblower complaint.

Darren Samuelsohn contributed to this report.