But to make its case, the United States attorney’s office leaned heavily on the testimony of Jona Rechnitz, a former donor to Mr. de Blasio, who has pleaded guilty and is cooperating with federal prosecutors.

Mr. Rechnitz testified to paying Mr. Seabrook $60,000 in cash stuffed inside a designer bag as payment for Mr. Seabrook’s investment of union funds.

Defense lawyers, however, assailed Mr. Rechnitz’s credibility, telling jurors that he was a pathological liar and that he continued to lie on the witness stand. Mr. Rechnitz gave the defense lawyers plenty of fodder for their accusations. He admitted telling a string of lies before signing his cooperation agreement, including to law enforcement, family members and potential investors in his various business ventures.

And he made no secret of his efforts to buy influence with key city policymakers — not only Mr. Seabrook, who was one of the city’s most powerful labor leaders for more than two decades, but also Mr. de Blasio.

What in particular has divided the jury is unknown. But some of the jurors appeared put off by Mr. Rechnitz at various points in his six days on the witness stand, despite prosecutors’ efforts to assure jurors that Mr. Rechnitz had no incentive to lie now that he had pleaded guilty.