President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE in a tweet early Thursday sought to cast doubt on impeachment witness David Holmes's allegation that he overheard a call between the president and U.S. Ambassador to the European Union Gordon Sondland Gordon SondlandGOP chairman vows to protect whistleblowers following Vindman retirement over 'bullying' Top Democrat slams Trump's new EU envoy: Not 'a political donor's part-time job' Trump names new EU envoy, filling post left vacant by impeachment witness Sondland MORE, raising the difficulty of eavesdropping on a phone conversation not on speaker.

"I have been watching people making phone calls my entire life. My hearing is, and has been, great," Trump tweeted just before Holmes, a U.S. Embassy staffer in Kyiv, Ukraine, made his opening statement ahead of testimony before the House Intelligence Committee.

"Never have I been watching a person making a call, which was not on speakerphone, and been able to hear or understand a conversation," Trump continued. "I’ve even tried, but to no avail. Try it live!"

I have been watching people making phone calls my entire life. My hearing is, and has been, great. Never have I been watching a person making a call, which was not on speakerphone, and been able to hear or understand a conversation. I’ve even tried, but to no avail. Try it live! — Donald J. Trump (@realDonaldTrump) November 21, 2019

The tweet was an indication that Trump is tuned in to Thursday morning's proceedings in the House impeachment inquiry. It is the fifth day of public testimony.

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Holmes, a State Department staffer working in Ukraine, has alleged that he overheard a phone conversation in July between Sondland and Trump. He has asserted that Trump asked Sondland about investigations and that the ambassador replied that the Ukrainian president would do "anything you ask him to" because "he loves your ass."

Asked about the latter comment on Wednesday, Sondland testified that it "sounds like something I would say."

Holmes told lawmakers in a private deposition that he could hear the discussion because the president was speaking loudly and Sondland was holding the phone away from his ear.

Democrats view Holmes as a key witness despite his late addition to the slate of public testimony, as his claims would tie Trump more directly to the push for investigations into his political rivals.