For the final stage of the prosecutor’s case in the People of the State of New York v. Donald Trump, the prosecution is expected to call the witness who has received the lion’s share of attention since the indictment was brought a year ago: Michael Cohen.

Before the trial started, some observers thought Mr. Cohen would be an indispensable star witness. They said that without Mr. Cohen, the district attorney could not establish the elements of the charged criminal offenses. But having seen the proof laid out meticulously and methodically by the prosecution these past three weeks, I find myself wondering: Do prosecutors even need Michael Cohen as a witness? Does the jury need to hear from him?

To be sure, the case would not exist but for Mr. Cohen. It is he who first revealed to prosecutors in the special counsel Robert Mueller’s office (I was one of the special counsel prosecutors) and in New York the hush-money scheme to buy Stormy Daniels’s silence in the aftermath of the “Access Hollywood” tape’s disclosure.

But since then, Alvin Bragg, the Manhattan district attorney, has amassed evidence that appears to independently both prove the crime and corroborate Mr. Cohen’s account.

To call Mr. Cohen as a witness carries with it not only the reward of adding further critical evidence to the prosecution’s case, but also the risk of undermining the case with issues related to Mr. Cohen’s personal baggage. He is, like Ms. Daniels, a colorful character, catnip to the press, and his broken bond with his former boss, Mr. Trump, is inherently dramatic.

He joins a line of famous underlings who turned state’s evidence against their boss. The prosecution will be relying on the testimony of an insider testifying “up,” against his boss. And what those insiders also have in common is that they can speak to the inner workings of an alleged conspiracy.

I have personally observed this in organized crime cases (Salvatore Gravano, the former underboss of the Gambino crime family, testifying against the former Gambino boss John Gotti), economic crime prosecutions (the former Enron chief financial officer Andrew Fastow testifying against the former chief executives Kenneth Lay and Jeffrey Skilling) and political corruption matters (Mr. Trump’s former deputy campaign manager Rick Gates testifying against the former campaign manager Paul Manafort).

Mr. Cohen can provide a detailed insider account of the charged scheme and Mr. Trump’s alleged role in it. And because the trial has already established that there is simply no alternative narrative that is consistent with all the trial proof, Mr. Cohen is far less important to its outcome than initially thought.

Testimony from witnesses like David Pecker, Jeffrey McConney and Hope Hicks has laid out a clear narrative of a scheme to kill derogatory stories about candidate Trump and disseminate damaging accounts about his political adversaries — including direct conversations with Mr. Trump. And smoking-gun handwritten notes of Trump Organization financial personnel provide evidence for the alleged cover-up of that scheme through documentation that disguises the reimbursement of the hush money as legal fees.

It is a sign of the unusual political dimension of this trial that Ms. Daniels, and not the far more legally damaging witnesses Mr. Pecker and Ms. Hicks, was subject to the far more intense cross-examination.

A key remaining issue — and one that Mr. Cohen can address — is whether Mr. Trump was aware of the alleged cover-up scheme involving reimbursement checks to Mr. Cohen disguised as legal payments. Mr. Cohen laid out the hush-money payments to Ms. Daniels by taking out a home-equity credit line for $130,000, a fact that was well established by direct and circumstantial evidence.

Take the handwritten notes from Allen Weisselberg, the former Trump Organization chief financial officer. They reveal that he must have been aware of the hush-money scheme and its alleged cover-up. His notes (and the notes of the former Trump Organization controller who prosecutors say helped to carry out the scheme) detail how to reimburse Mr. Cohen the $130,000 the amount would need to be doubled, or “grossed” up, to account for taxes on the amount of this disguised income. The idea that Mr. Weisselberg, a Trump Organization veteran who has apparently been willing to serve time in jail rather than turn against Mr. Trump and remained on the Trump payroll even after his guilty pleas, would have approved these payments on his own is far-fetched. Trial evidence establishes that he could not approve expenses over $10,000, and here he would be approving not just the payment of $130,000 to Mr. Cohen, but doubling it to make him whole.

Witness after witness — as well as Mr. Trump’s own words read to the jury — attest to his being both a micromanager and a penny-pincher. And Mr. Trump signed check after check reimbursing Mr. Cohen for what he paid Ms. Daniels plus much more. In short, it seems implausible that either Mr. Cohen or Mr. Weisselberg would dare such a move without Mr. Trumps’ permission.

Against all this evidence and more, we are now expected to hear from Mr. Cohen. Successful prosecutions can often not be made without such witnesses. This is particularly true when the bosses consciously do not leave a paper trail. Mr. Trump famously groused about his White House counsel taking notes, observing that one of his favorite lawyers, Roy Cohn, never did.

So no doubt Mr. Bragg and his prosecutors believe they must call Mr. Cohen to testify. Still, calling Mr. Cohen as a witness does carry significant risk for Mr. Bragg. He brings baggage: He recently claimed under oath in a New York civil fraud trial against Mr. Trump (where the court found him credible and ruled against Mr. Trump) that he lied to a federal judge when he pleaded guilty to one of several crimes. By way of explanation, he seemed to contend he was pressured to plead guilty by the federal prosecutors.

Even accepting Mr. Cohen’s story, it means he lied to a federal judge after taking an oath to tell the truth — the same oath he will take at the criminal trial of Mr. Trump. And his story would support an anticipated defense claim that the federal prosecutors were so intent on making a case against Mr. Trump that they were willing to trample on Mr. Cohen’s rights — and that ugly federal muck will splatter on the state prosecutors.

The other option is that Mr. Cohen is lying about not being guilty of the charge — which may be a very distinct possibility given the proof against him. If that is the case, it would mean he lied in the recent state court fraud case. As a federal judge in New York recently concluded in denying Mr. Cohen’s motion for early termination of his criminal sentence, he lied in one forum or the other.

Still, that does not mean Mr. Cohen should not be called. I have repeatedly observed an interesting phenomenon in cases in which the prosecution has a mountain of independent evidence of guilt, but still calls a flawed insider to provide unique detailed and direct evidence to the jury of the defendant’s guilt.

Jurors often want to hear someone recount what they already know occurred, but that has not been said directly. The jurors will then often reach a verdict of guilty, and despite having found the conspiracy existed as recounted by a key criminal accomplice — someone like Mr. Cohen — they will later say they did not believe or need that witness’s testimony.

We will soon learn whether that will happen again in the People v. Trump.

Andrew Weissmann teaches at the N.Y.U. School of Law and is a co-author of “The Trump Indictments: The Historic Charging Documents With Commentary.” He was a senior prosecutor in Robert Mueller’s special counsel investigation and is a co-host of the podcast “Prosecuting Donald Trump.”

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