An aide to Rudy Giuliani claims President Trump "knew exactly what was going on" regarding efforts to pressure Ukraine into investigating former Vice-President Joe Biden and his son.
Lev Parnas, a close associate of Mr Trump's lawyer, made the comments in an interview with MSNBC.
Mr Parnas, who is facing separate criminal charges, said Mr Giuliani was never investigating corruption.
The intention was to damage a potential Democratic rival to President Trump.
The president denies the allegations.
Donald Trump was impeached in the US House of Representatives last month and is accused of abuse of power and obstruction of Congress. The case was sent to the Senate on Wednesday.
Mr Parnas said that the president is lying and knew that unless there was an investigation into Mr Biden and his son Hunter, who was a director of a Ukrainian gas company, military aid would be withheld to Ukraine.
He also claimed that the investigation was about digging dirt on Mr Biden, who is seeking the Democratic nomination to challenge Donald Trump in this year's US presidential election.
Earlier this week, letters, phone records, notes and flash drives were obtained from Mr Parnas, a Ukranian – American businessman, in a bid to bolster Democrats' case against Mr Trump at the forthcoming Senate trial.
Documents show that Ukraine-born Mr Parnas was in regular contact with Mr Giuliani as well as Ukrainian officials, and suggest that Mr Parnas was directly involved in trying to have Ukraine's President Volodymyr Zelensky look into Mr Biden.
What did Lev Parnas say?
In the interview aired on Wednesday, Mr Parnas said Mr Trump was "aware of all my movements".
"I wouldn't do anything without the consent of Rudy Giuliani or the president. Why would President Zelensky's inner circle or [Interior] Minister [Arsen] Avakov or all these people or [former] President [Petro] Poroshenko meet with me?
"Who am I? They were told to meet with me. And that's the secret that they're trying to keep. I was on the ground doing their work," he added.
What new evidence was obtained from Mr Parnas?
One handwritten note from Mr Parnas states: "Get Zalensky [sic] to Annouce [sic] that the Biden case will be investigated."
Also among the new materials is a screenshot of a previously undisclosed letter from Mr Giuliani to Mr Zelensky, in which he asks to arrange a meeting.
The letter from Mr Giuliani describes himself as "personal counsel to President Trump" and states that Mr Trump had "knowledge and consent" of Mr Giuliani's actions.
The meeting never took place as Mr Giuliani eventually cancelled his May trip to Ukraine.
What about the former US ambassador?
Some of the materials obtained show Mr Parnas and Mr Giuliani discussing the removal of Marie Yovanovitch, the then US ambassador to Ukraine.