David Holmes, the counselor for Political Affairs at the U.S. embassy in Ukraine, arrives to appear for a closed-door deposition as part of the impeachment inquiry into President Donald Trump on Capitol Hill in Washington, November 15, 2019.

According to testimony Holmes gave to House investigators last week, he personally heard President Donald Trump speaking to the U.S. ambassador to the European Union, Gordon Sondland, during a lunch he and Sondland attended in late July.

David Holmes, an American diplomat stationed in Ukraine, will testify before the House Intelligence Committee Thursday starting at 9:00 a.m., alongside former National Security Council aide and Russia expert Dr. Fiona Hill.

House Democrats on Monday added a last-minute witness to the lineup of individuals scheduled to testify in the ongoing impeachment inquiry this week.

Holmes testified that he heard Trump ask Sondland whether Ukrainian President Volodymyr Zelenskiy had agreed to launch investigations into Trump's political rivals.

'"So, he's gonna do the investigation?" Holmes said he heard Trump ask on July 26, one day after the president had spoken with Zelenskiy by phone. During that call, Trump asked the newly elected Ukrainian leader to investigate the son of former U.S. Vice President Joe Biden.

"He's gonna do it," Sondland replied to Trump, according to an opening statement Holmes gave to House investigators. Holmes also testified that Sondland went on to tell the president that Zelenskiy would do "anything you ask him to."

While it's not clear what else Holmes told investigators during his deposition on Friday, he could nonetheless prove to be an important witness for Democrats, because he can testify that he personally heard the president ask about Ukraine investigations.

Several of the witnesses in the impeachment inquiry so far have described events and conversations that they learned about from others, given that they did not have direct access to the president.

This has given Trump and his Republican allies an opening to claim that the allegations being leveled against the president are based on second hand information, and "hearsay."

"Another day, another Democrat star witness with no firsthand knowledge and all hearsay," Trump's 2020 campaign spokeswoman Kayleigh McEnamy wrote on Twitter Friday.

This week's hearings, however, will feature testimony from more than half a dozen current and former Trump administration officials who have firsthand knowledge of Trump's effort to pressure Ukraine to investigate his rivals.

The House impeachment probe focuses on whether Trump abused the power of his office by withholding congressionally appropriated foreign aid to Ukraine, while his personal lawyer, Rudy Giuliani, worked with a small group of Trump aides to get Ukraine's president to launch the investigations that Trump wanted.

WATCH: Holmes full statement during impeachment inquiry