Mr. López Obrador pursued the case in earnest after winning a landslide victory in 2018, as prosecutors zeroed in on a transfer of $3.6 million into a shell company tied to Mr. Lozoya, which they said was facilitated by Odebrecht. After fleeing the country, Mr. Lozoya was arrested in Spain in February and extradited to Mexico, where he is cooperating with authorities.
He has said that the millions distributed to campaign advisers at the direction of Mr. Peña Nieto and Mr. Videgaray came from bribes paid by Odebrecht, according to the attorney general.
The investigation could bolster an administration in dire need of a political victory. Mr. López Obrador remains popular here, but his poll numbers have been declining in the midst of a pandemic that has left more than 53,000 dead and millions out of work. Congressional elections will take place next year and the president’s party will have to fight to maintain its control of the legislature.
But making such explosive accusations public before any formal charges are filed comes with its own risks. Mexican prosecutors have a history of bungling high-profile investigations and using them for political ends, and if the new revelations are mishandled or lead nowhere, it could hurt the president in the long run, analysts said.
“It worries at least some of us that the president only wants to win the elections and not actually investigate corruption,” said Esteban Illades, a newspaper columnist. “He definitely needs a win, because we are on the road to 60,000 deaths here in Mexico.”
Mr. López Obrador has repeatedly expressed skepticism about prosecuting former presidents, and suggested at a news conference on Wednesday that he would seek a referendum before going after Mr. Peña Nieto. He also seemed to play down the consequences of the allegations.
“If it was just a statement, without evidence, it has no legal, and I would say, social and moral value,” Mr. López Obrador said. “There has to be proof to back it up, evidence, witnesses. There is apparently a video, I would want to see it, like all Mexicans.”
Paulina Villegas contributed reporting.