Giuliani said his lawyer Jon Sale sent a letter to Congress stating he will not comply with the subpoena seeking documents related to a phone call between Trump and Ukrainian President Volodymyr Zelensky.
"Please accept this response as a formal notice that Mr. Giuliani will not participate because this appears to be an unconstitutional, baseless and illegitimate 'impeachment inquiry,'" the letter stated.
Giuliani added that he was no longer maintaining Sale's services and would hire a new lawyer if Congress seeks to enforce the subpoena.
"If they enforce it, then we will see what happens," he told ABC News.
Tuesday was the deadline for Giuliani to comply with the subpoena from House Intelligence Chairman Adam Schiff, House Foreign Affairs Chairman Eliot Engel and House Oversight Chairman Elijah Cummings.
The subpoena cited a "growing public record" including Giuliani's own comments that indicate he and Trump pressured the Ukrainian government to pursue two "politically motivated investigations."
"The first is a prosecution of Ukrainians who provided evidence against Mr. Trump's convicted campaign chairman, Paul Manafort. The second relates to former Vice President Joseph R. Biden Jr., who is challenging President Trump for the presidency in 2020," it stated.