Not too long ago, President Donald Trump aimed a lot of derision at Michael Cohen and Paul Manafort for being “flippers,” the word he used for people who change their loyalties to assist government prosecutors in exchange for more lenient treatment for themselves. “Flipper” is a term that lies somewhere between the snarled mob word, “snitch,” and the prosecutor’s more court-appropriate term, “cooperating defendant.”

This week, the special counsel's office said Manafort lied to the FBI and investigators, breaking his plea agreement. While we don't have all the details yet, this newest development is actually a good illustration of why using cooperating defendants is not an abuse of prosecutorial power.

The New York Times and a third rate reporter named Maggie Haberman, known as a Crooked H flunkie who I don’t speak to and have nothing to do with, are going out of their way to destroy Michael Cohen and his relationship with me in the hope that he will “flip.” They use.... — Donald J. Trump (@realDonaldTrump) April 21, 2018

Many different kinds of witnesses might be called to testify in a criminal trial. Depending on the trial, jurors might hear from fact witnesses, percipient witnesses, expert witnesses, character witnesses, cooperating witnesses, and cooperating defendants. Cohen and Manafort fall into the last category — people who were charged with crimes and who subsequently agreed to provide information and testify for the government to help the prosecution of others. Usually this help is offered in exchange for a reduction of charges, sentences, monetary penalties and sometimes even to get more lenient treatment for others, such as a family member who was also caught up in the crime.

While Trump rants about many things, his accusation that having defendants testify for the prosecution in exchange for leniency “ought to almost be illegal” and “almost ought to be outlawed” is creating ripple effects in the judicial community. Some defense attorneys are already working the president’s statement into closing arguments for their own clients; prosecutors in the ongoing Joaquin “El Chapo” Guzman drug cartel trial have asked the judge not to allow defense attorneys to argue that cooperating witnesses were “flippers” who even the president of the United States said should not be believed.

But the bottom line is that cooperation is just fine, and it certainly ought not be outlawed.

While Trump rants about many things, his accusation that having defendants testify for the prosecution in exchange for leniency “ought to almost be illegal” and “almost ought to be outlawed” is creating ripple effects in the judicial community.

Criminal defendants usually have three criticisms of cooperators: First, that cooperation is unfair because it gives the government an advantage that defendants (or potential defendants) don’t have; second, that cooperation is unseemly because it rewards disloyalty by the cooperator to former friends and business associates; and third, that the information and testimony given by the cooperator is unreliable because it is motivated by self-interest.

It’s not entirely clear whether Trump objects to cooperating defendants on one, two, or all of these grounds, but numbers one (unfairness) and two (disloyalty) are not legitimate gripes. After all, when it comes to fairness, most prosecutors think it’s the criminals who have the advantage. Criminals have endless time to plan, execute, and cover up their crimes; they also have the advantage of surprise, because they get to choose when to commit their crime. Unconstrained by ethical obligations, criminals have also been known to intimidate witnesses. Manafort was himself accused of witness tampering, and the Wall Street Journal reported earlier this month that Mueller was investigating whether Trump confidante Roger Stone had attempted to intimate witnesses as well.

And with regard to loyalty, “honor among thieves” is simply not a legal right.

That leaves what appears to be the president’s primary complaint: that a cooperator’s information is unreliable because it’s motivated by self-interest. That concern is not entirely without merit. In fact, a variation of this complaint actually received some air time 20 years ago, when a federal appeals court agreed with a convicted felon, Sonya Singleton, who argued a deal between federal prosecutors and a defendant who testified against Singleton amounted to “bribery” under the federal anti-bribery statute. Based on this conclusion, the three-judge court reversed Singleton’s conviction.

Citing the federal bribery law that “Whoever…directly or indirectly, corruptly gives, offers, or promises anything of value to any person…with intent to influence the testimony…” is guilty of a crime, the three-judge panel held that the government’s promise of leniency to the cooperator, in exchange for testimony, did in fact violate the bribery statute. But when all 12 judges of the court re-heard the case (a procedural step reserved for cases of unusual importance), the full court held that the bribery statute does not apply to government prosecutors who offer leniency in exchange for truthful cooperation. Singleton’s conviction was affirmed.

That was the right result. In fact, the reason testimony by a cooperating defendant should not be “outlawed” is because there is nothing “corrupt” about such deals. A violation of the bribery statute means that compensation was paid corruptly, and corruption requires some element of deception, falsehood or error. But deals made by prosecutors always see the light of day; a prosecutor has to tell a defendant about any promises or compensation it gave a witness. The defendant is then free to argue to the jury — as the president has to the public — that the witness must be lying. The full airing in court of the government’s promises to the witness means that any concerns about credibility go, in court parlance, “to the weight of the evidence, not its admissibility.” In other words, the jury gets to hear the cooperator’s testimony, but the jury also gets to decide how much weight to give that testimony in light of the leniency the cooperator will receive.

So Robert Mueller’s team must tell a defendant about any promises they’ve made to any witness in exchange for cooperation, and that’s why the special counsel’s deals with Manafort and Cohen are not illegal. Any agreements made by the special counsel are documented and clearly disclosed — and can be used by the defense to challenge the credibility of those cooperators.

Prosecutors are also careful to make clear to cooperators that their deals depend on complete and truthful testimony. (I have had cooperating defendants who, while testifying against former cohorts, described their own participation in drug deals, fraudulent schemes, and even killings, sometimes in extraordinarily colorful language). When presenting the testimony of a cooperating defendant, it’s important to corroborate, corroborate and corroborate again that testimony with irrefutable facts. (Some states even require corroboration of a cooperator’s testimony).

Every prosecutor knows that without corroboration in the form of documents, surveillance, or other witness testimony, the testimony of a cooperating defendant is severely compromised. Corroborating evidence is critical; ideally, the cooperator’s testimony merely fills in the blanks.

And finally, if the cooperator gives information that turns out to be demonstrably untruthful or incomplete, the prosecutor has the ability to nullify the deal and not only revoke any promises of leniency, but also reinstate any charges that might have been dismissed under the plea deal. This guarantees the prosecutor the benefit of the bargain; it’s also a powerful incentive for a cooperating witness to be completely truthful and forthcoming.

This is a test that the special counsel believes Paul Manafort has ultimately failed to meet. But because Manafort and his lawyers dispute assertions that he lied or broke the plea deal, it will likely be up to the court to decide.

The president’s grumblings aside, it’s unlikely that “flipping” will go away any time soon. Sometimes, to get to the truth, a prosecutor has no choice but to strike a deal. Cooperation will always be a necessary tool in the prosecutor’s toolbox because, as many a prosecutor has explained in closing argument, “Crimes conceived in hell don’t have angels as witnesses.”