Dramatic testimony from U.S. diplomats working in Ukraine have significantly raised the stakes for this week’s impeachment inquiry appearance from Gordon SondlandGordon SondlandSondland notified Trump officials of investigation push ahead of Ukraine call: report Impeachment hearings don’t move needle with Senate GOP Lawmakers spar over upcoming Sondland testimony MORE, the mega-donor to President TrumpDonald John TrumpTrump reversed course on flavored e-cigarette ban over fear of job losses: report Trump to award National Medal of Arts to actor Jon Voight Sondland notified Trump officials of investigation push ahead of Ukraine call: report MORE who is now the U.S. ambassador to the European Union.

Sondland is expected to come under tough questioning from Democrats and Republicans on the House Intelligence Committee on Wednesday after shifting his initial statement in the inquiry to acknowledge it was his belief that Trump linked Ukrainian security assistance to that country announcing investigations of former Vice President Joe BidenJoe BidenSondland notified Trump officials of investigation push ahead of Ukraine call: report Biden says he won’t legalize marijuana because it may be a ‘gateway drug’ Impeachment hearings don’t move needle with Senate GOP MORE and his son Hunter.

Testimony last week from William Taylor, the top U.S. diplomat in Ukraine, has also put a new spotlight on Sondland. Taylor testified that one of his staffers overheard Sondland speaking with Trump about the desired investigations into Biden and 2016 election meddling.

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The staffer, David Holmes, then testified about what he heard to House lawmakers in a closed-door deposition on Friday. In his opening statement, first reported by CNN, Holmes said that he heard Trump ask if Ukrainian President Volodymyr Zelensky is “gonna do the investigation,” to which Sondland replied that Zelensky will do “anything you ask him to.”

Tim Morrison, the top Russia adviser on the National Security Council, also revealed in closed-door testimony unveiled over the weekend that Sondland said he was getting direction from Trump to push for the investigations. He will take the stand the day before Sondland, adding pressure on the ambassador to clear up his side of the story.

Democrats have expressed unhappiness with Sondland’s changing story and questioned his credibility.

“I think he shaves a lot of truth from his answers and I think he’s going to have to pay for it. He certainly is going to have to come in and try and rehabilitate himself. But there’s probably going to be questions on both sides of the aisle as to whether or not he’s capable of telling the truth,” said Rep. Jackie SpeierKaren (Jackie) Lorraine Jacqueline SpeierOvernight Defense: Ex-Ukraine ambassador offers dramatic day of testimony | Talks of ‘crisis’ at State Department | Trump tweets criticism of envoy during hearing | Dems warn against ‘witness intimidation’ | Trump defends his ‘freedom of speech’ Democrats say Trump tweet is ‘witness intimidation,’ fuels impeachment push Democrats seize on new evidence in first public impeachment hearing MORE (D-Calif.), an Intelligence Committee member.

Republicans may do little to defend the ambassador — who is one of the few witnesses who has spoken directly to Trump about the aid to Ukraine.

Rep. Mark MeadowsMark Randall MeadowsDemocrats seize on new evidence in first public impeachment hearing House Republicans call impeachment hearing ‘boring,’ dismiss Taylor testimony as hearsay Key takeaways from first public impeachment hearing MORE (R-N.C.), a top Trump ally who sat in on Sondland’s deposition as a member of the House Oversight and Reform Committee, described Sondland as “the wild card for everybody.”

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“They’re all wondering what Sondland’s going to say and how he’s going to say it,” Meadows said.

GOP lawmakers have repeatedly faulted impeachment witnesses so far for their second- or third-hand accounts of Trump’s Ukraine policies, but Sondland potentially offers a chance to cut through the game of “telephone.”

“He’s important because he’s a first-person witness, and he is a direct link to the president. So he’s going to be very important,” said Rep. Jim HimesJames (Jim) Andres HimesLawmakers spar over upcoming Sondland testimony Sunday shows — Spotlight shifts to Sondland ahead of impeachment inquiry testimony Himes: ‘I don’t think it blows a hole in the case’ if Sondland testifies there was no quid pro quo MORE (D-Conn.), a member of the Intelligence Committee.

The challenge, Himes said, will be to pin Sondland down on precise details. In his closed-door testimony last month, Sondland repeatedly responded to investigators’ questions by saying that he did not recall events or details that allegedly transpired.

“It is interesting that Sondland’s testimony in the deposition was perhaps not complete. What we don’t have evidence of is of him outright lying,” Himes said. “I think that imposes a burden on us to be particularly specific in the questioning.”

Republicans repeatedly dismissed testimony last week from Taylor, as well as George Kent, the deputy assistant secretary of State for European and Eurasian Affairs, as only second-hand with limited direct knowledge of Trump’s efforts to pressure the Ukrainian government to open the investigations he wanted.

Taylor revealed during his testimony before the cameras that one of his staffers overheard a phone call between Sondland and Trump on July 26, the day after Trump spoke with Zelensky about looking into the Biden family.

But Meadows predicted that Sondland would not offer evidence of Trump telling him that the aid to Ukraine was conditioned on opening the investigations into the Bidens and a conspiracy theory about 2016 election interference.

“I think what you’ll find is that anytime he had direct contact with the president of the United States, and not Rudy GiulianiRudy GiulianiSondland notified Trump officials of investigation push ahead of Ukraine call: report Impeachment hearings don’t move needle with Senate GOP Want to fight foreign influence? Fix FARA filings MORE or others, that the message was clear: There was no condition placed on aid, nor was there any conditionality expected,” Meadows said.

Still, Meadows downplayed the centrality of Sondland’s involvement despite his firsthand interactions with Trump.

“I don’t see Ambassador Sondland as being the key to whether this president is impeached or not,” he said.

Sondland originally testified during a closed-door deposition in mid-October that he did not definitively know why the aid to Ukraine was withheld. He also told lawmakers that he asked Trump in an early September phone call what he wanted from Ukraine. Trump, according to Sondland, said: “I want no quid pro quo. I want [Ukrainian President Volodymyr] Zelensky to do the right thing.”

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“And I said: ‘What does that mean?’ And he said: ‘I want him to do what he ran on.’ And that was the end of the conversation. I wouldn’t say he hung up on me, but it was almost like he hung up on me,” Sondland added.

But Sondland later revised his testimony after reviewing statements from other witnesses that “refreshed [his] recollection about certain conversations.” Sondland stated that he “presumed” that the suspension of military aid to Ukraine was in fact linked to the Ukrainian government announcing the desired investigations.

He also said that he told a top adviser to Zelensky that the aid would likely not resume until Ukraine moved forward with the probes.

“I now recall speaking individually with Mr. [Andrey] Yermak, where I said that resumption of U.S. aid would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks,” Sondland said.

The inconsistency has raised eyebrows in both parties.

“He has some questions to answer with regards to the addendum that he submitted,” said Rep. Lee ZeldinLee ZeldinFive takeaways from ex-ambassador’s dramatic testimony Ex-Ukraine ambassador testifies in impeachment hearing: Five things to watch Latest impeachment developments: Republicans huddle ahead of Friday’s hearing MORE (R-N.Y.), a Foreign Affairs Committee member who attended Sondland’s closed-door deposition.

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Sondland won’t be the only witness with direct knowledge of Trump’s actions on Ukraine to testify this week.

Lt. Col. Alexander VindmanAlexander VindmanImpeachment hearings likely to get worse for Republicans Himes: ‘I don’t think it blows a hole in the case’ if Sondland testifies there was no quid pro quo Trump opens new line of impeachment attack for Democrats MORE, the director for European Affairs at the National Security Council, and Jennifer Williams, a foreign service officer assigned to Vice President Pence’s office, were both on the July 25 call between Trump and Zelensky and will testify on Tuesday morning.

Vindman also testified that he heard Sondland say during a meeting that “the Ukrainians would have to deliver an investigation into the Bidens.”

Later Tuesday, former special envoy to Ukraine Kurt VolkerKurt VolkerWhite House releases rough transcript of early Trump-Ukraine call minutes before impeachment hearing Haley: Giuliani should’ve been named ‘special envoy’ to Ukraine Overnight Energy: Perry replacement faces Ukraine questions at hearing | Dem chair demands answers over land agency’s relocation | Ocasio-Cortez, Sanders unveil 0B Green New Deal public housing plan MORE, who worked closely with Sondland and participated in a May meeting with Trump, as well as Morrison, will testify before the public.

After Sondland testifies on Wednesday, the House Intelligence Committee will hear from Defense Department official Laura Cooper and David Hale, the under secretary of State for political affairs. 

Fiona Hill, the former National Security Council senior director for Europe and Russia, is scheduled to be the last to testify publicly on Thursday. 

—Mike Lillis contributed.