ATLANTA — A former friend and colleague of the Fulton County district attorney, Fani Willis, testified Thursday that Ms. Willis’ romantic relationship with a special prosecutor began before she hired him to lead the election interference case against President Trump.

Robin Yeartie’s testimony directly contradicts Ms. Willis and Nathan Wade’s statements about their relationship, threatening to upend the case accusing Mr. Trump and others of conspiring to overturn the 2020 presidential election results in Georgia. 

Mr. Wade, who was ordered to take the witness stand, also acknowledged that he had sex with Ms. Willis during his separation from his estranged wife, even though he had claimed that wasn’t the case in divorce filings.

Mr. Wade said his relationship with Ms. Willis ended last summer but they remain good friends, adding that they are “probably closer than ever because of these attacks.” Mr. Wade said their relationship was not a secret — just private — because they are “private people.”

“There is nothing secret or salacious about having a private life,” Mr. Wade said. “Nothing.”

An attorney representing Mr. Trump’s co-defendant Michael Roman, Ashleigh Merchant, has described the relationship as a conflict of interest that should disqualify Ms. Willis — and her entire office — from the case. 

If that were to happen, a council that supports prosecuting attorneys in Georgia would find a new attorney to take over who could either proceed with the charges against Mr. Trump and 14 others or drop the case altogether.

Ms. Merchant alleges that Ms. Willis personally profited from the case, paying Mr. Wade more than $650,000 for his work and then benefiting when Mr. Wade used his earnings to pay for vacations the pair took together.

The judge refused to quash a subpoena for Mr. Wade and he took the witness stand after Ms. Yeartie’s testimony. He insisted that the pair didn’t start dating until after he was hired as special prosecutor in 2022. 

He also testified that he and Ms. Willis traveled together to Belize, Aruba, and California and took cruises together, but said Ms. Willis paid him back in cash for some travel expenses that he had charged to his credit card.

“She was very emphatic and adamant about this independent, strong woman thing so she demanded that she paid her own way,” Mr. Wade said.

Mr. Wade was pressed by defense attorneys to answer uncomfortable questions about his relationship with Ms. Willis, prompting objections from the district attorney’s office. The hearing began with lengthy sparring between lawyers over who must answer questions. It is expected to stretch into Friday.

Ms. Willis could also be forced to testify in the hearing though her lawyers are fighting to keep her off the witness stand.

Ms. Willis’ removal would be a stunning development in the most sprawling of the four criminal cases against Mr. Trump. Even if a new lawyer went forward with the case, it would very likely not go to trial before November, when Mr. Trump is expected to be the Republican nominee for president. 

In a court filing earlier this month, Ms. Willis’ office insisted that she has no financial or personal conflict of interest and that there are no grounds to dismiss the case or to remove her from the prosecution. 

Her filing called the allegations “salacious” and said they were designed to generate headlines. Mr. Wade said in an affidavit filed in court that their relationship began after he was hired and that they have never lived together.

But Ms. Yeartie, who worked in the district attorney’s office until early last year, testified that she saw Mr. Wade and Ms. Willis hugging and kissing prior to November 2021.

Since the allegations of an inappropriate relationship surfaced, Mr. Trump has used them to try to cast doubt on the legitimacy of Ms. Willis’ case against him. Other Republicans have cited them in calling for investigations into Ms. Willis, a Democrat who’s up for re-election this year.

Mr. Roman’s lawyer, Ms. Merchant, subpoenaed Ms. Willis, Mr.  Wade, seven other employees of the district attorney’s office, and others, including Mr. Wade’s former business partner, Terrence Bradley. 

Mr. Bradley took the witness stand earlier Thursday but refused to answer questions from Ms. Merchant, citing attorney-client privilege.

Judge Scott McAfee said during a hearing Monday that Ms. Willis could be disqualified “if evidence is produced demonstrating an actual conflict or the appearance of one.”

The judge said the issues he wants to explore at the hearing are “whether a relationship existed, whether that relationship was romantic or nonromantic in nature, when it formed and whether it continues.” 

Those questions are only relevant, he said, “in combination with the question of the existence and extent of any personal benefit conveyed as a result of the relationship.”