Prosecutors investigating whether Donald Trump committed crimes as he sought to overturn his 2020 election defeat in Georgia are running into increasing resistance as they seek to call witnesses to testify before a special grand jury.

The latest illustration of that came Wednesday, when lawyers for Republican Gov. Brian Kemp filed a motion to quash a subpoena for his testimony, accusing the office of Fulton County District Attorney Fani Willis, a Democrat, of pursuing his testimony for “improper political purposes.” Willis rejected that characterization, describing it as dishonest.

Kemp is just one of several witnesses who have pushed back against Willis’ attempt to compel their testimony in a case investigating potential criminal interference in an election. Late Wednesday, U.S. Sen. Lindsey Graham formally appealed a judge’s order requiring him to testify before the special grand jury on Aug. 23. And John Eastman, a conservative lawyer who aided Trump’s efforts to undo the 2020 election results, has also pushed back against his subpoena, with a judge in New Mexico on Wednesday rejecting his request and ordering him to travel to Atlanta to testify before the special grand jury.

The witnesses’ reluctance to testify in the case reflects the high stakes of the investigation, which is just one of a long list of serious legal threats that Trump is facing that have intensified in recent weeks. It also demonstrates the power that Trump continues to wield over the Republican Party as he prepares for an expected 2024 presidential campaign.