Terrence Bradley, an Atlanta-area lawyer, had been billed as the star witness in the effort to disqualify Fani T. Willis, the district attorney leading the election interference case against former President Donald J. Trump in Georgia. But when Mr. Bradley took the stand this week — and twice earlier this month — he was a deeply reluctant witness.

His testimony did little to resolve a question at the heart of the defense’s attempt to show that Ms. Willis had an untenable conflict of interest: Whether the romantic relationship between Ms. Willis and Nathan Wade, the lawyer she hired to help run the Trump case, began before or after he joined her staff.

But hundreds of text messages obtained by The New York Times show that Mr. Bradley, a former law partner and friend of Mr. Wade, helped a defense lawyer to expose the relationship between the two prosecutors.

The texts reveal that Mr. Bradley, who served for a time as Mr. Wade’s divorce lawyer until the two men had a bitter falling-out, assisted the effort to reveal the romance and provide details about it for at least four months — countering the impression he left on the witness stand that he had known next to nothing about the romance.

In the text exchanges, which began in September of last year and were reported late Wednesday by CNN, Mr. Bradley claimed some knowledge of when the relationship began. He even offered the defense lawyer, Ashleigh Merchant, reassurance as she submitted her motion to disqualify to the court.

“I am nervous,” Ms. Merchant texted to Mr. Bradley on Jan. 8, the day she filed the motion. “This is huge.”