A new investigation unfolding at the Georgia State Capitol is previewing the turbulence that Fani T. Willis, the district attorney prosecuting Donald J. Trump, may face even if a judge allows her to keep the high-stakes case.

At a hearing on Wednesday before a special committee of the State Senate, which recently began investigating Ms. Willis, the defense lawyer leading the disqualification effort testified that Ms. Willis had once taken a large sum of money from her political campaign for her personal use.

The senator presiding over the hearing, Bill Cowsert, a Republican, was taken aback by the allegation. “I think all of us up here that have campaigns know that you can’t take campaign funds for personal use, right?” he asked.

The defense lawyer, Ashleigh Merchant, told Mr. Cowsert that she was “not well versed in that area of law, but that is my understanding.” He replied, “It’s pretty black and white.”

There is no evidence to back up the allegation. In fact, Ms. Willis lent her first campaign nearly $50,000, drawn from a retirement account, and was paid back only a fraction of that, according to her office, campaign finance records and her past remarks.