The top Democrat on the House Intelligence Committee on Monday subpoenaed Rudy Giuliani, President Donald Trump's personal lawyer, for documents related to House Democrats' impeachment inquiry investigating allegations that Trump used his office to pressure Ukraine's leader to look into a political opponent.

House Intelligence Committee Chairman Adam Schiff of California, in consultation with House Foreign Affairs Committee Chairman Eliot Engel of New York and House Oversight and Reform Committee Chairman Elijah Cummings of Maryland, sent a letter to Giuliani requesting that he produce those documents by Oct. 15. The three chairmen also requested documents and depositions with three of Giuliani's business associates: Lev Parnas, Igor Fruman and Semyon "Sam" Kislin.

Giuliani has emerged as a central figure in the growing controversy, especially after the release of an intelligence community whistleblower's complaint alleging that Trump demanded Ukrainian President Volodymyr Zelenskiy to investigate potential corruption by former Vice President Joe Biden and his son Hunter Biden. A five-page memo of the July 25 call released by the White House revealed Trump asked Zelenskiy to get in touch with Giuliani and Attorney General William Barr.

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"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 letter to Giuliani reads.

"Our inquiry includes an investigation of credible allegations that you acted as an agent of the president in a scheme to advance his personal political interests by abusing the power of the office of the president."

The letter also cited a television appearance from Sept. 19 when CNN anchor Chris Cuomo asked Giuliani if he asked Ukraine to investigate Biden. Giuliani initially denied it, saying he asked the country to investigate unsubstantiated claims that the Ukranians interfered in the 2016 election. But when pressed again, Giuliani responded, "Of course I did."