WASHINGTON — Only a handful of subordinates to a U.S. president have ever found themselves in the unenviable position of deciding whether to publicly implicate the commander in chief in impeachment proceedings — John Dean, Monica Lewinsky and others whose names are seared into American history.

No one, however, has faced quite the dilemma now confronting Gordon Sondland.

The evidence gathered to date points to Sondland as the witness who, more than any other, could tie President Donald Trump directly to the effort to persuade Ukraine to launch investigations that might benefit him politically.

On Wednesday, with cameras rolling, the millionaire Republican donor-turned-ambassador could solidify the case against Trump, though doing so would require that he revise his previous testimony or acknowledge significant omissions. Or he could stand by his statements and face withering questioning from Democrats on the House Intelligence Committee over inconsistencies between his testimony and that of a growing number of witnesses.

"The impeachment effort comes down to one guy, Ambassador Sondland," said Rep. Mark Meadows, R-N.C., who like many Republicans has argued that only a first-person account of Trump leveraging U.S. power for personal gain could give Democrats grounds to impeach. "All the other testimony has a Sondland core to it and a Sondland connection."

Sondland's future — and possibly his freedom — could also rest on whether lawmakers believe he is telling the whole truth about his role and that of the president. Lawmakers in previous inquiries have referred witnesses to the Justice Department if they believe they have lied under oath.

One Republican appeared to raise such a possibility in a brief interview Monday night — particularly if Sondland backs away from his testimony that Trump did not direct a quid pro quo.

"I expect Ambassador Sondland to tell us the same thing he said in his deposition," said House Intelligence Committee member Michael Conaway, R-Texas. Asked what would happen if he does not, he said: "Well, there are legal ramifications for that, for changing your [testimony]. He's got to have good reasons."

Sondland's potential legal exposure is rooted in seven hours of closed-door testimony he provided to congressional investigators Oct. 17. Sondland said then that he had little contact with Trump and knew of no link between a freeze on U.S. aid to Ukraine and investigations sought by Trump into the energy company Burisma, where former vice president Joe Biden's son held a board position, or into a widely discredited theory that Ukraine had circulated misinformation to influence the 2016 U.S. presidential election.

Sondland told Congress that he would not have assisted in any effort by the president to press Ukraine to investigate a potential 2020 challenger and that he would have viewed such an effort as inappropriate.

Already, Sondland has reversed himself on a key point. In supplemental testimony, he wrote that the accounts of witnesses who testified after him had "refreshed" his memory.

Though he previously said he knew of no link, Sondland wrote that on Sept. 1 he warned a top Ukrainian official that $400 million in U.S. assistance would probably flow to the country only if its president publicly promised to launch the investigations. Sondland said he had come to "presume" that the White House had linked the aid to the investigations and so shared that presumption with the Ukrainians.

In recent weeks, additional inconsistencies have emerged between Sondland's account and those of at least a half dozen other Foreign Service and national security officials, all of whom will testify publicly before Congress by the end of the week.

Witnesses have said Sondland pressured Ukrainian officials over the investigations, including at a White House meeting July 10. Sondland last month said he recalled no such exchange.

The prospect that Sondland could further revise his initial testimony and more directly implicate the president — or that he could hold fast — adds an element of unpredictability to an already unprecedented impeachment investigation.

Where the probes into President Richard Nixon's culpability in the Watergate scandal and President Bill Clinton's lies about an affair with an intern were recounted in lengthy reports compiled by special prosecutors, Attorney General William Barr's dismissal of concerns about Trump's actions left Democratic lawmakers to conduct their own investigation. As a result, the Trump inquiry from the start has played out almost in real-time and often on national television.

Timothy Naftali, a history professor at New York University and coauthor of the book "Impeachment," said past probes into scandals that threatened U.S. presidencies contain few parallels to Sondland's upcoming testimony.

When former White House counsel John Dean testified against Nixon in 1973, he did so under a plea agreement that sent him to prison for obstruction of justice. When national security officials Oliver North and John Poindexter testified in televised hearings regarding the Iran-Contra affair, under President Ronald Reagan, both had struck immunity agreements in advance that would later keep them out of jail.

Sondland is testifying with no such safety net.

"It's really an amazing moment — the first time an impeachment investigation has evolved in real time," Naftali said. And because Trump has so far blocked acting chief of staff Mick Mulvaney and others closest to him from testifying, Sondland has emerged as the witness with potentially the most knowledge of the president's involvement in the matter.

"There is going to be enormous pressure on Sondland to minimize the president's role — even as we have people under oath saying Sondland has already misrepresented his position and the extent of the contacts he had with the president," Naftali said.

Indeed, no other impeachment witness has stirred up so much anxiety on Capitol Hill during the impeachment inquiry. Ahead of Sondland's testimony, both sides were engaged in a sort of tug-of-war trying to win Sondland to their side, while also coming up with contingency plans for what to do if he does not.

While Republicans have said they do not expect Sondland to change his testimony, Democrats have sought in public statements to encourage him to feel comfortable revising his testimony.

"If I was Ambassador Sondland, I would take a deep breath and start over," said Rep. Mike Quigley, D-Ill. "Take into consideration everything we have learned from other witnesses. Tell us exactly what took place and his interactions with the president … It's never too late to do the right thing."

Rep. Eric Swalwell, D-Calif., a former prosecutor, used similarly encouraging words. "It would be liberating for you to just be truthful," he said in an interview. "It's amazing how many people rise to the occasion to do that. The fact that [Sondland] amended his testimony should be encouraging."

At the same time, Democrats sought to downplay Sondland’s testimony ahead of the hearing, lest he refuse to confirm his conversations with the president — or say he lied or exaggerated to colleagues about how much he was in touch with Trump.

Perhaps the most important discrepancy that has emerged is the number of contacts Sondland, the ambassador to the European Union, had with Trump. Sondland testified that they spoke or met just three times from late May to early September, the time frame in which the White House sought to press Ukrainian President Volodymyr Zelensky to publicly commit to the two political investigations.

Last week, David Holmes, a counselor at the U.S. Embassy in Ukraine, told Congress that he and others witnessed a fourth interaction, a July 26 call Sondland placed from a Kyiv restaurant. Sitting at an outdoor table, as waiters passed by, Holmes said Sondland told him and two other aides that he was going to update the president, then called the White House on his cellphone.

Holmes said he remembers the moment vividly for the colorful language Sondland used with the commander in chief. At one point, Holmes testified, Sondland told Trump that “President Zelensky ‘loves your ass.’ ”

Trump's voice could initially be heard across the table, Holmes said, because it was so loud that Sondland grimaced and pulled the receiver away from his year.

"I then heard President Trump ask, 'So, he's gonna do the investigation?' " Holmes testified, according to a transcript. "Ambassador Sondland replied that 'he's gonna do it,' adding that President Zelensky will do 'anything you ask him to.' "

Holmes testified that after the call, he asked Sondland whether it was true that Trump did not care about Ukraine.

“Nope, not at all, doesn’t give a s— about Ukraine,” Holmes recalled Sondland saying. "I asked why not, and Ambassador Sondland stated, the President only cares about ‘big stuff.’ I noted that there was ‘big stuff’ going on in Ukraine, like a war with Russia.' Sondland said ‘no, big stuff that matters to him, like this Biden investigation that Giuliani is pushing.’ "

Other witnesses have said Sondland was speaking about a Biden investigation well before the call.

On July 10, Sondland went off-script at a meeting with Ukrainian officials, according to testimony from White House Russia adviser Fiona Hill and White House Ukraine adviser Army Lt. Col. Alexander Vindman.

At the White House meeting, Sondland pressed a top aide to Zelensky and the head of Ukraine's national security council for "investigations into the 2016 election, the Bidens and Burisma," Vindman testified. Hill and Vindman said they confronted Sondland afterward.

Sondland testified he has no recollection of making such demands or of having tense words with Hill or Vindman afterward.

Tim Morrison, the top Russia and Europe adviser on the National Security Council, testified that in all, he understood Sondland had spoken to Trump about half a dozen times from mid-July to mid-September. Sondland has acknowledged only two calls with Trump in that time.

Trump tweeted about Sondland last month, calling him a “great American” when the State Department initially instructed him to decline to testify without a subpoena. Asked about Sondland this month, as the details of his testimony fell into dispute, Trump said, “I hardly know the gentleman.”