After weeks in which President Trump’s top aides have figured as the major players in the Ukraine narrative, testimony in the first few hours of the public impeachment hearings Wednesday thrust Trump himself back to center stage.

Acting ambassador to Ukraine William B. Taylor Jr. told lawmakers about a previously unknown effort by the president to make sure Ukraine was looking into his political opponents: a phone conversation he said Trump had with a top U.S. diplomat asking about the status of “the investigations.”

The phone conversation described by Taylor gave Democrats a chance to renew questions about Trump’s personal involvement in the effort to push Ukraine to investigate his political opponents while the United States withheld security assistance and a sought-after White House meeting.

Rep. Peter Welch (D-Vt.) said he would be “glad to have the person who started it all come in and testify.”

“President Trump is welcome to take a seat right there,” Welch said, gesturing to the witness desk.

On Wednesday afternoon, Trump denied any knowledge of the phone call described by Taylor.

“I know nothing about that,” Trump said during a White House news conference with Turkish President Recep Tayyip Erdogan. “First time I’ve heard it. . . . I don’t recall, not at all, not even a little bit.”

Before Wednesday, the president had surfaced in two key moments: a May 23 Oval Office meeting, in which he told U.S. officials to confer with his personal attorney Rudolph W. Guiliani on Ukraine policy, and a July 25 phone call when he asked Ukrainian President Volodymyr Zelensky to investigate Democrats.

Taylor kicked off the first public hearing of the impeachment inquiry by describing another act by Trump — a phone conversation he said the president had with U.S. Ambassador to the European Union Gordon Sondland a day after the call between the two presidents.

Taylor told lawmakers that his aide was accompanying Sondland during a July 26 visit to Kyiv when, in a restaurant, Sondland phoned Trump to update him about meetings he was having in the city, including a sit-down with a top Zelensky adviser.

Taylor said that the aide overheard Trump ask Sondland about “the investigations” and that Sondland told the president the Ukrainians were “ready to move forward.”

Taylor said his aide later asked Sondland what Trump thought about Ukraine. Sondland said that Trump cared “more about the investigations of Biden, which Giuliani was pressing for,” Taylor testified.

Sondland told reporters in Ukraine on July 27 that he spoke to Trump after the Zelensky call, but it is unclear if he was referring to the same conversation that Taylor described.

Sondland attorney Robert Luskin said in an email to The Washington Post that his client “will address any issues that arise from this in his testimony next week,” when he is scheduled to testify in another open hearing.

Taylor’s account of the call undercuts Trump’s recent claims that he doesn’t know Sondland, whom he called a “really good man and great American” in a tweet last month.

But last week, Trump told reporters at the White House, “Let me just tell you: I hardly know the gentleman.” The president’s comment came after Sondland reversed his previous testimony in the impeachment inquiry, acknowledging that he told a Ukrainian official in September that U.S. military aid probably would resume only if the Kyiv government opened Trump’s desired investigations.

Taylor said he had not provided his account to investigators during his Oct. 22 closed-door deposition because his staff member told him about the episode only last Friday.

“I am including it here for completeness,” Taylor said of the information, adding that “it is my understanding that the committee is following up on this matter.”

Taylor said he also reported the story through counsel to the House Intelligence Committee and the State Department’s legal adviser.

The aide who overheard Sondland’s call with the president is embassy staffer David Holmes, who will testify Friday behind closed doors in the House’s impeachment probe, according to two people familiar with the investigation. Holmes is the top political affairs officer at the U.S. Embassy in Ukraine.

Holmes received an award from theAmerican Foreign Service Association in 2014 for speaking up internally against the Obama administration’s policy on Afghanistan, potentially complicating any Republican plans to paint him as a liberal partisan. The William R. Rivkin Award acknowledges a mid-level Foreign Service officer and recognizes the value of dissent, according to the American Foreign Service Association journal.

Trump’s first known role in the effort to pressure Ukraine took place in the Oval Office meeting May 23, when he instructed a U.S. delegation that had just returned from Zelensky’s inauguration to “talk to Rudy,” referring to Giuliani, about Ukraine policy, according to witness testimony.

“Rudy had some bad issues with Ukraine, and until Rudy was satisfied, the president wasn’t going to change his mind,” Sondland said in a closed-door deposition.

Two months later, during the July 25 phone call, Trump told Zelensky that “whatever you can do” to investigate former vice president Joe Biden and his son Hunter Biden “would be great” and urged him to speak with Giuliani and Attorney General William P. Barr.

“We will get to the bottom of it,” Trump said. “I’m sure you will figure it out.”

Correction: A previous version of this report stated incorrectly that Holmes had received the William R. Rivkin award from the State Department. The award came from the American Foreign Service Association.

Aaron C. Davis, Josh Dawsey, Karoun Demirjian, Karen DeYoung and Anne Gearan contributed to this report.