William Taylor, the top U.S. diplomat in Ukraine, linked President Donald Trump closer to the effort to get the country to probe his political opponents in an open congressional hearing on Wednesday.

Taylor — the star witness House Democrats were pushing for to testify as part of their impeachment inquiry — provided an extensive and detailed explanation of Trump’s “highly irregular” foreign policy in Ukraine. More damningly, he revealed that one of his staffers listened to a conversation Trump and U.S. Ambassador to the European Union Gordon Sondland had just one day after the president’s infamous July 25 phone call with the leader of Ukraine. According to Taylor, the aide claimed Trump wanted updates on “the investigations,” a reference to the inquiries that the president’s personal lawyer Rudy Giuliani pushed for into former Vice President Joe Biden and other prominent Democrats.

The U.S. chargé d’affaires for Ukraine went on to say that Sondland told Trump during their conversation over the summer that Ukraine was “ready to move forward” with the probes. The diplomat added that the ambassador to the E.U. explicitly told Taylor’s staffer that Trump “cares more about the investigations of Biden” than about giving military aid to Ukraine.

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Taylor’s revelations help boost Democratic impeachment investigators’ case that Trump abused his power by enlisting Ukraine’s help to probe his political rivals for personal gain. The diplomat said on Wednesday he did not disclose his staffer’s account of the conversation between Trump and Sondland in his closed-door deposition last month because he did not know about it. Democrats have now called for Taylor’s staff member to testify.

House Intelligence Committee Chairman Adam Schiff (D-California) stated Taylor’s updated testimony bolsters Democrats’ theory that Trump ordered both Giuliani and Sondland to pressure Ukraine to investigate his political opponents.

Trump denied the conversation with Sondland in comments to reporters at the White House on Wednesday.

“First time I heard it,” Trump said in a news conference with Turkish President Recep Tayyip Erdogan.