Gordon Sondland’s testimony before the House Intelligence Committee on Wednesday could be the most important in the impeachment inquiry. The U.S. ambassador to the European Union is a witness who can concretely tie a bribery scheme to President Trump. But only if he tells the truth about conversations that he had with Trump — conversations that other witnesses have described but which Sondland has not previously told Congress about.

The hearings so far have revealed that U.S. officials sought an announcement by the Ukrainian government that it was conducting investigations that would implicate Trump’s political rivals, and that officials used as leverage $400 million approved by Congress for military aid to repel Russia. Trump and his supporters have attempted to distance the president from the scheme. It’s the classic “I didn’t know” defense of company executives, elected officials, organized crime bosses and heads of drug cartels that we often saw as federal prosecutors.

Trump’s defenders suggest that the president was simply trying to root out corruption in Ukraine before handing over the money. If anyone pressured Ukraine to do anything improper, that was the work of rogue actors like Trump’s personal lawyer Rudolph W. Giuliani and acting White House chief of staff Mick Mulvaney, the president’s defenders say. Trump and his supporters have dismissed damaging witness testimony by career diplomats and intelligence professionals as “hearsay” — even though the rough transcript the White House released of the July 25 call with Ukrainian President Volodymyr Zelensky clearly shows Trump asking for investigations of the Biden family and the 2016 election as “a favor.”

But Sondland is a direct witness whose testimony on Wednesday could upend claims that Trump was an innocent bystander. Other testimony from multiple U.S. officials suggests that Sondland spoke directly to Trump about his demand for a public announcement about investigations into his political rivals and that Sondland relayed to Ukrainian officials that, without such an announcement, they would not get the military aid or the White House meeting they wanted.

Sondland’s testimony about these matters could strongly bolster the case that Trump demanded a personal political favor in exchange for performing official duties, an abuse of power that could rise to the level of an impeachable offense.

As Sondland prepares for his day at the witness table, he faces self-inflicted challenges. He has already testified before the committee in a closed-door deposition. In that deposition, Sondland denied participating in any conversations with Ukrainian officials about exchanging investigations for military aid. He later submitted supplemental written testimony admitting that he had participated in such discussions. This revision came only after his memory had been “refreshed” by the conflicting testimony of other witnesses.

But even after that, testimony emerged that suggests Sondland may have failed to disclose another key fact when he denied that he had discussed the matter with the White House or State Department. David Holmes, a State Department official has testified about an incident in which Sondland spoke to Trump by phone from a Kyiv restaurant. During the call, Trump spoke loudly enough for everyone at the table to hear, said Holmes, who was present. According to Holmes, Trump asked Sondland about the investigations, and Sondland told Trump that Ukraine was ready to move forward with them. Holmes further testified that after the call, Sondland said that Trump did not care about Ukraine, only about announcing an investigation against a political rival.

If that’s true, Sondland’s testimony Wednesday could link the president directly to the investigations-for-aid scheme. Based on our experience dealing with witnesses in criminal cases who have been contradicted by other testimony, there are three likely scenarios for how the morning could go.

First, Sondland could double down on his prior testimony and persist in his failure to recall his conversations with Trump. If he lies Wednesday, he could face prosecution for false statements or perjury — in the short term, that would require Trump’s allies at the Justice Department to bring charges against someone for allegedly covering for the president, but if there were a new administration in 2021, Sondland would be at risk of prosecution. Last week’s guilty verdict against Trump confidant Roger Stone should provide a cautionary tale about the consequences of standing by lies in a public proceeding.

Second, Sondland could invoke his Fifth Amendment right against self-incrimination and refuse to testify. While all witnesses have the right not to incriminate themselves, witnesses like Sondland in the public arena risk the damaging perception that they believe that they have engaged in criminal conduct. Also, in Sondland’s case, he likely has largely waived his Fifth Amendment right as to the substantive topics by testifying previously about the same subject matter he will be called upon to discuss. But truthful testimony now could potentially expose him to charges of perjury for past lies, and his attorney could argue that means his right not to testify are preserved.

His third option is to tell the truth. This would be, in our view, his best choice. A defense against perjury is to recant a false statement during the same “proceeding.” Although some could contend that Sondland’s deposition is a different proceeding from the public hearings, it could easily be characterized as part of the same proceeding for purposes of the law, and so it is not too late for Sondland to come clean.

If Sondland contradicts his past deposition and confirms his phone call with Trump, Republicans will almost certainly attack his credibility. Democrats, in turn, will ask House members to believe a witness who has now changed his story more than once. This could be harmful to their case but not necessarily fatal.

As prosecutors, we took witnesses as we found them. In a perfect world, all witnesses would tell the truth the first time. In reality, such witnesses don’t often participate in “drug deals” of a real or political nature. Witnesses who have firsthand knowledge of bad conduct have often engaged in bad conduct themselves. We frequently had to deal with witnesses who had information about a crime only because they associated with the subject of an investigation into the crime in the first place. Prosecutors often say that they didn’t choose the witness, the defendant did.

So how can anyone believe a witness who appears to have already lied? Neither prosecutors — nor, in this case, House investigators — can just take a witness’s word for it. Instead, they need other reliable evidence to corroborate the testimony. Here, corroboration for Sondland’s testimony comes in the form of testimony by other credible witnesses, Sondland’s text messages with acting ambassador to Ukraine William B. Taylor Jr., the rough transcript of the July 25 phone call containing Trump’s own words, and public statements by Mulvaney, who admitted that Trump demanded the investigations before releasing the military aid.

Another way to assess the veracity of a witness’s testimony is by asking whether he has a motive to tell the truth or to lie. Sondland had a motive to minimize harm to himself and Trump when he testified earlier that he did not recall key facts. In fact, Sondland’s alleged coverup show a consciousness of guilt that the goal he and Trump were after was not to fight corruption generally — because if it were, he would have admitted to his conduct in the first place. Exposing the demand that Ukraine announce an investigation in exchange for military aid would be against Sondland and his boss’s interests, and so admitting to it now would have the ring of reliability.

The public shouldn’t necessarily expect a made-for-TV moment where Rep. Adam B. Schiff (D-Calif.), chairman of the House Intelligence Committee, or lead committee counsel Daniel Goldman (skilled as they both are) get Sondland to admit “I lied to protect the President!” In real life, admissions come in small bites, step by step when confronted with facts that are inescapable — like a phone call so loud it was overheard by nonparticipants in a restaurant. To steer Sondland toward the truth, House Democrats would be wise to confront him with the contradictory testimony of other witnesses and ask him whether they are true or false. Some witnesses are willing to shade or evade, but will tell the truth when confronted with a stark choice between facts and lies.

It is not too late for Sondland to explain what happened. Telling the truth is in his own best interests — and the best interests of the country.