Gordon Sondland, U.S. ambassador to the European Union, arrives to the Capitol for his deposition as part of the House's impeachment inquiry on Thursday, Oct. 17, 2019.

A top Trump donor and top Trump administration diplomat could become a top witness for House Democrats Wednesday, when he testifies publicly as part of the impeachment inquiry into President Donald Trump.

U.S. ambassador to the European Union Gordon Sondland is set to appear for a public hearing Wednesday at 9 a.m. ET before the House Intelligence Committee, chaired by Rep. Adam Schiff, D-Calif.

Two more witnesses — Defense Department official Laura Cooper and State Department official David Hale — are slated to appear for a joint public hearing before the Intelligence panel Wednesday afternoon.

Sondland dealt with Trump more closely than any other witness who has so far publicly testified in the impeachment probe, which centers around the efforts to get Ukraine to announce investigations involving Trump's own political rivals.

While a transcript of Sondland's closed-door testimony from mid-October has already been made public, it's far from clear what he will say in his public hearing.

That's partly because Sondland returned to Capitol Hill in early November to make a major revision to his testimony that appears to bolster the allegation that Trump pressured Ukraine to commit to the political probes.

In an addendum to his testimony, Sondland admitted that he told a top Ukraine official that hundreds of millions of dollars in military aid to Ukraine — which had been withheld without clear explanation at the time from the Trump administration — would likely not be delivered until Ukraine agreed to make a "public anti-corruption statement" that had been under discussion.

Other witnesses have also testified about Sondland's interactions with Trump — and raised questions about his testimony.

David Holmes, political counselor at the U.S. Embassy in Kyiv, testified that he overheard a July 26 conversation between Trump and Sondland — one day after Trump asked Ukraine President Volodymyr Zelenskiy to "look into" unsubstantiated allegations about the Bidens and the 2016 election.

In the call with Sondland, Trump asked if Zelenskiy is "gonna do the investigation," Holmes testified. Sondland, speaking from Kyiv, said Zelenskiy is "gonna do ... anything you ask him to," according to Holmes' testimony.

The Washington Post notes that Sondland's testimony states that he recalls "no discussions with any State Department or White House official about former vice president Biden or his son. Nor do I recall taking part in any effort to encourage an investigation into the Bidens."

In the afternoon hearing, Cooper, deputy assistant secretary of defense for Russia, Ukraine and Eurasia, is expected to testify that the military aid allocated to Ukraine by Congress had been held without explanation. The delay began to raise concerns among administration officials, Cooper said. That aid package was eventually released in mid-September

Hale, the undersecretary of state for political affairs, will testify alongside Cooper.