A Georgia appellate court said Thursday that Fulton County District Attorney Fani Willis and her team should be removed from the election subversion prosecution of President-elect Donald Trump, likely dealing a knockout blow to the case.

The bombshell decision by the Georgia Court of Appeals upends one of the few remaining cases against Mr. Trump, who will be inaugurated on Jan. 20.

The panel concluded that Ms. Willis’ romantic relationship with Nathan Wade, a paid investigator on the case, was an impermissible financial conflict.

A trial judge in Atlanta had allowed Ms. Willis to stay on the case as long as Mr. Wade departed, but attorneys for Mr. Trump and his codefendants appealed.

“After carefully considering the trial court’s findings in its order, we conclude that it erred by failing to disqualify DA Willis and her office,” the panel of three Republican-appointed judges wrote.

The judges said booting Mr. Wade from the case “did nothing” to address the appearance of impropriety.

Ms. Willis began investigating Mr. Trump three years ago based on his efforts to pressure Georgia Secretary of State Brad Raffensperger to gather enough votes to overturn what Mr. Trump called a rigged 2020 victory in the state for Joseph R. Biden.

A grand jury indicted Mr. Trump and his associates on racketeering charges and other counts.

The appellate ruling is a massive blow to the case and a win for Mr. Trump, who weathered legal strife in 2022 and 2023 before romping through the Republican primary race and riding outrage about inflation and illegal immigration to victory over Vice President Kamala Harris in November.

Mr. Trump’s team said Ms. Willis’ case against the president-elect should be dismissed entirely.

“In granting President Trump an overwhelming mandate, the American people have demanded an immediate end to the political weaponization of our justice system and a swift dismissal of all the witch hunts against him,” Trump spokesman Steven Cheung said. “We look forward to uniting our country as President Trump Makes America Great Again.”

The Georgia case was hobbling along while special counsel Jack Smith moved to dismiss two federal cases against Mr. Trump, citing precedents that say sitting presidents cannot be criminally prosecuted.

Ms. Willis dug in and said her case should move forward, but a high court said she could not proceed. Georgia’s Prosecuting Attorneys’ Council will appoint a new attorney if the case survives.

The charges could be dropped if a district attorney does not want to take over Ms. Willis’ case.

That happened with Georgia Lt. Gov. Burt Jones, whom Ms. Willis charged with trying to overturn the 2020 election results. Mr. Jones planned to sign a certificate stating that Mr. Trump had won the state’s electoral votes.

Ms. Willis was removed from the prosecution after it was revealed that she had hosted a fundraiser for Mr. Jones’ Democratic opponent. The alternate prosecutor decided not to pursue the trial.

Mr. Trump’s attorney, Steve Sadow, said the case against the president-elect must die.

“As the court rightfully noted, only the remedy of disqualification will suffice to restore public confidence,” Mr. Sadow said. “This decision puts an end to a politically motivated persecution of the next president of the United States.”

Trump allies said the decision shouldn’t be the end of the matter. They want Ms. Willis to be punished.

Lawyer Mike Davis, a Republican operative who leads the Article III Project against “leftist lawfare,” said Ms. Willis took illegal kickbacks.

“Fani lied about them under oath. Now Fani got disqualified. And Fani must go to prison,” Mr. Davis said.

Elsewhere, a New York judge is figuring out how to deal with Mr. Trump’s convictions of falsifying business records. A jury found him guilty in the hush-money case this year, though sentencing was delayed because of the presidential campaign and Mr. Trump’s victory.

Prosecutors want to preserve the conviction but delay sentencing until 2029 or put the case on hold.

The Willis matter in Georgia seemed to come out of nowhere at the start of the year.

A motion filed by Ashleigh Merchant, an attorney for Trump co-defendant Michael Roman, said she learned of the Willis-Wade relationship through multiple sources.

The issue gained momentum, resulting in a multiday hearing that was must-see TV for political observers.

Ms. Willis and Mr. Wade said they were not dating when Mr. Wade was hired, but defense attorneys said cellphone records and witness accounts seemed to indicate otherwise.

Ms. Willis acknowledged her relationship with Mr. Wade but said that was no reason to disqualify her, noting they maintained separate residences and bank accounts. They said they broke up when the matter exploded into public view.