The former president of Mexico, Enrique Peña Nieto, took a $100 million bribe from Joaquín Guzmán Loera, the infamous crime lord known as El Chapo, according to a witness at Mr. Guzman’s trial.

The stunning testimony was delivered Tuesday in a New York courtroom by Alex Cifuentes Villa, a Colombian drug lord who worked closely with Mr. Guzmán from 2007 to 2013, when the kingpin was hiding from the law at a series of remote ranches in the Sierra Madre mountains.

"Mr. Guzmán paid a bribe of $100 million to President Peña Nieto?” Jeffrey Lichtman, one of Mr. Guzmán’s lawyers, asked Mr. Cifuentes during cross-examination.

"Yes,” Mr. Cifuentes said.

Mr. Guzmán may offer more details soon. Shortly after the jury was excused around 4:30 p.m. Tuesday, Mr. Lichtman submitted his client’s name to the prosecution as a potential witness for the defense, confirming that the drug trafficker might testify in his own trial.

Mr. Lichtman said that adding Mr. Guzman’s name to the witness list does not guarantee that he will testify. It is simply "possible.”

"If I didn’t put him on the list, it would possibly foreclose the possibility for him to testify,” Mr. Lichtman said in an interview. "So, I was just being inclusive.”

Mr. Guzmán’s testimony would be a stunning development. While his lieutenants have shared details about the Sinaloa cartel’s operations, the kingpin himself could offer even more intimate information, such as how he possibly bribed a president of Mexico.

According to Mr. Cifuentes, Mr. Peña Nieto first reached out to Mr. Guzmán about the time he was elected president in late 2012, asking the drug lord for $250 million in exchange for calling off a nationwide manhunt for him.

But Mr. Guzmán made a counteroffer, Mr. Cifuentes added, saying he would give Mr. Peña Nieto only $100 million.

"The message was that Mr. Guzmán didn’t have to stay in hiding?” Mr. Lichtman asked.

"Yes,” Mr. Cifuentes said, "that very thing is what Joaquin said to me.”

Mr. Lichtman, quoting Mr. Cifuentes’s notes from an interview he gave to American authorities in 2016, asked whether Felipe Calderón, who preceded Mr. Peña Nieto as Mexico’s president, took a bribe in 2008 from one of Mr. Guzmán’s rivals, the Beltrán-Leyva brothers.

"I don’t recall this incident very well,” Mr. Cifuentes answered. He added moments later, "Right now, I do not remember that.”

Mr. Peña Nieto and Mr. Calderón could not yet be reached for comment.

While other witnesses at Mr. Guzmán’s trial in Federal District Court in Brooklyn have testified about huge payoffs from traffickers to the Mexican police and public officials, the testimony about Mr. Peña Nieto was the most egregious allegation yet. If true, it suggests that corruption by drug cartels had reached into the highest level of Mexico’s political establishment.

After testifying about the two presidents, Mr. Cifuentes rattled off other bribes that Mr. Guzmán and his allies had paid to Mexican officials. On at least two occasions, he said, the kingpin gave the Mexican military between $10 million and $12 million to launch operations to "either kill or capture” associates of the Beltrán-Leyva brothers during his war with them.

Mr. Cifuentes also said the Mexican federal police not only turned a blind eye to drug trafficking, but occasionally took part in it. Once, he told jurors, traffickers gave the police photographs of several suitcases packed with cocaine that were sent by the cartel on an airplane from Argentina to Mexico. The police picked up the suitcases from the baggage claim, Mr. Cifuentes said, and sold the drugs themselves.

All of this came on Mr. Cifuentes’s exhausting second day as a witness at Mr. Guzmán’s trial. He has already confessed to a staggering array of crimes.

On the stand, Mr. Cifuentes admitted to hatching a failed murder plot with the Hell’s Angels in Canada. He acknowledged buying plastic explosives from the widow of a Honduran drug trafficker. He said he paid a judge in Ecuador $500,000 to throw out the case of an Ecuadorean military officer accused of working with the cartel, adding that he later helped kidnap the officer when it seemed that he was cheating Mr. Guzmán.

There were lurid hints that top Mexican leaders might have been compromised by dirty money from the start of the trial in November. In his opening statement, Mr. Lichtman claimed his client had been framed for years by a conspiracy hatched by his partner, Ismael Zambada García, in league with "crooked” American drug agents and a "completely corrupt” Mexican government, including two of its presidents.

At the time, Mr. Peña Nieto and Mr. Calderón released statements calling the accusations false. The judge in the case, Brian M. Cogan, later cautioned Mr. Lichtman against making promises to the jury that the evidence in the case would not support.

Then, as the first week of the trial came to an end, Mr. Guzmán’s lawyers informed Judge Cogan at a sidebar conference that a coming witness, Jesus Zambada García, Ismael Zambada’s brother, would testify, if asked, that Mexican presidents had taken bribes from the Sinaloa drug cartel.

But Judge Cogan forbade the testimony, citing the embarrassment it would cause to unnamed "individuals and entities” who were not directly involved in the case.

On Tuesday, however, Judge Cogan allowed Mr. Cifuentes to testify about what he knew concerning bribes to Mexican presidents with only a few interruptions.

At one point, under questioning by Mr. Lichtman, Mr. Cifuentes acknowledged that his personal assistant, Andrea Velez Fernandez, had worked for a political consultant, J.J. Rendón, who was hired by Mr. Peña Nieto’s presidential campaign. Mr. Cifuentes said Ms. Velez had once sent him photographs of "suitcases filled with cash.”

When Mr. Lichtman asked if the suitcases were "destined for Mr. Peña Nieto,” prosecutors objected on the grounds of relevance.

"Agreed,” Judge Cogan said.

After that, there was no more discussion of the suitcases.

Agencies