Sondland also flummoxed Trump's current top envoy to Ukraine, Bill Taylor, when he asked colleagues to exclude notetakers from a June call with Zelensky. And he later relayed to Taylor, who testified last week, that "everything" — a White House visit and military aid — depended on the opening of Trump's favored investigations.

If the testimony from the seven witnesses who have addressed the publicly underscore anything, it's that Sondland can pull together all of the far-flung elements of Democrats' impeachment investigation and provide clarity — or sink the probe into further confusion — with his testimony on Wednesday.

And despite the high expectations for Sondland, there’s reason to be wary. The testimony to this point has raised serious questions about Sondland’s credibility. Some witnesses nicked Sondland for boasting about his close ties to Trump and said they wondered at times whether it was puffery.

Morrison said Tuesday that when he succeeded Hill in her senior position on the National Security Council, she warned him to beware of "The Gordon Problem.” It was a reference to Sondland’s intense effort to secure the politically motivated Ukraine investigations for Trump, he recalled.

But Morrison and others also attested that when Sondland claimed to speak to Trump, he truly had spoken to Trump.

Unlike many of the other witnesses who have already testified, Trump hasn't attacked Sondland, who won the EU ambassadorship after contributing $1 million to the president’s inaugural committee.