A judge in Fulton County, Georgia, has granted a motion by a Republican candidate for lieutenant governor to disqualify District Attorney Fani Willis from targeting him in her criminal probe into former President Donald Trump and allies’ efforts to overturn the results of the 2020 election.

Superior Court Judge Robert McBurney on Monday barred Willis from building a case against Burt Jones, a state senator running for lieutenant governor, after she hosted a fundraiser backing Democratic candidate Charlie Bailey in his runoff against another Democratic challenger who ultimately lost, Kwanza Hall.

McBurney said Willis’ taking part in the fundraiser was “harmful” to the integrity of the investigation.

“Any decision the district attorney makes about Senator Jones in connection with the grand jury investigation is necessarily infected by it,” the judge wrote, adding that any effort to focus specifically on Jones, even if it were justified, would prompt “entirely reasonable concerns of politically motivated persecution.”

Willis’ interest in Jones stems from some state Republicans’ efforts after the 2020 election to create an alternate slate of 16 presidential electors even after an official vote count showed Trump had lost by thousands of votes. Jones is one of the “fake electors” named as targets in the investigation.

McBurney said an additional decision by Willis to donate privately to Bailey’s campaign was not disqualifying in itself, as Jones’ counsel had argued, but he said the move “added to the weight of the conflict created by the more extensive, direct, public and job-related campaign work the district attorney performed on behalf of candidate Bailey.” 

“This choice — which the district attorney was within her rights as an elected official to make — has consequences. She had bestowed her office’s imprimatur upon Senator Jones’s opponent,” he added. 

A special grand jury that convened in January began receiving evidence in the investigation last month, and Willis has issued subpoenas to at least a dozen of the false electors, including Jones. He joined 11 of the fake electors in filing a motion to quash their grand jury subpoenas. While McBurney granted Jones’ motion to disqualify Willis from investigating him, the judge denied the effort to bar Willis from investigating the others.

McBurney said his decision means Willis cannot subpoena Jones, publicly categorize him as a target or ask the grand jury to include recommendations about him in its final report. While Willis can gather evidence of Jones’ involvement in efforts to undermine the 2020 election results, she cannot use such evidence to build a case against Jones, the judge said. Any decision on charges would be left to a different prosecutor’s office, McBurney wrote.

NBC News has asked Willis’ office for comment.