Former Donald Trump lawyer Sidney Powell has pleaded guilty to six charges in the Georgia election interference case.
Powell, who pushed false claims about the 2020 election results, reached a deal with prosecutors and will now testify at the trial.
There are 18 co-defendants, including the former president, in the case.
She faced charges of conspiracy to commit intentional interference of election duties.
In exchange for her sentence of six years of probation, Powell must testify truthfully at future trials - a huge win for prosecutors in the sweeping election-fraud case, given her intimate involvement in Mr Trump's fight to overturn the state's election results.
The deal also required her to record a proffer, essentially giving a verbal account of all her activities in the case, and to pay a fine and write an apology letter to Georgia citizens.
Powell entered her guilty plea in a downtown Atlanta courtroom on Thursday, a day before her trial was set to start.
Prosecutors have accused her of being among a group of Trump officials and supporters who breached the elections system in rural Coffee County, Georgia, in January 2021, with the goal of proving that the election was rigged against Mr Trump.
Specifically, Powell was accused of hiring a forensics team and sending them to Coffee County to illegally access government computers to look at voter data.
Prosecutors had said she conspired to tamper with voting machines, electronic ballots and voter data, have ballots taken from a polling place, and stop an election worker from doing her job.
The ex-Trump lawyer was one of the most visible, and vocal, supporters of Mr Trump's false claims on the 2020 election.
She notably appeared at a press conference with other Trump lawyers in November 2020, where she alleged, without evidence, that US voting machines - run by Dominion Voting Systems - can be hacked to "take a certain percentage of votes from President Trump and flip them to President (Joe) Biden".
At a White House meeting the following month, Mr Trump mulled appointing Powell as a special counsel to "investigate allegations of voter fraud in Georgia and elsewhere", prosecutors said.
The congressional committee investigating the 6 January Capitol riot featured her prominently in its televised hearings last year, showing how she worked closely with Mr Trump and his aides on strategies to keep him in the White House.
Dominion has sued Powell for defamation, seeking $1.3 bn (£1.7 bn) in restitution.
Mr Trump has pleaded not guilty in his Georgia election fraud case. In total, the former president faces 13 felony counts - including racketeering - for allegedly pressuring Georgia officials to reverse the results of the 2020 presidential election in that state.
He has repeatedly denied any wrongdoing, and has described the case as being politically motivated.
Powell is the second person among the defendants to plead guilty in the Georgia election interference case.
Bail bondsman Scott Hall struck a plea deal with prosecutors in late September.
The former Republican poll watcher is also accused of trying to gain access to sensitive election equipment in Coffee County, Georgia.
As part of the plea deal, he was sentenced to five years probation. He is also required to testify against others in future trials.