Just two weeks after Donald J. Trump lost the 2020 election, the lawyer Sidney Powell rallied to the cause of keeping him in office during a news conference at the headquarters of the Republican National Committee.
Standing next to Rudolph W. Giuliani, one of Mr. Trump’s closest allies, Ms. Powell laid out a preposterous conspiracy theory. She told the world that a voting machine company called Dominion had worked with a liberal financier and Venezuelan intelligence to flip votes away from Mr. Trump to his opponent, Joseph R. Biden Jr.
The baseless assertions ultimately lay at the heart of a series of federal lawsuits that Ms. Powell filed challenging Mr. Trump’s defeat. And even though the Trump campaign later disavowed her, chiding her claims as unbelievable, she soon returned to Mr. Trump’s orbit, taking part with him in an Oval Office meeting to discuss a brazen plan to seize the country’s voting machines and effectively rerun the election.
On Thursday, in a move that caught the former president and his advisers by surprise, Ms. Powell pleaded guilty to election interference charges in Georgia and agreed to testify against the other defendants in the case — Mr. Trump among them.
The unexpected development was a significant victory for Fani T. Willis, the district attorney of Fulton County, Ga., who filed the election case this summer. But it was arguably even more important for Mr. Trump, marking the first time that anyone who was closely tied to his attempts to stay in power had reached a cooperation deal with the authorities.
It remains unclear what Ms. Powell might say about Mr. Trump if called upon to testify against him. But if she takes the stand in his election trial in Georgia, she could shed light on a number of gambits he undertook to stay in power despite the will of the voters.
Word of her agreement, which emerged without warning during a court hearing in Atlanta on Thursday, raised other questions as well: Would she also lend her help to the federal prosecutors who filed their own election case against Mr. Trump in Washington — one in which she appeared as an unindicted co-conspirator? And would any other figures in the case be open to accepting similar deals with prosecutors? (Ms. Powell pleaded guilty to misdemeanor charges and was sentenced to probation.)
Two people with ties to Mr. Trump’s orbit suggested that Ms. Powell might be a more problematic trial witness than it seems, given her history of outlandish statements. Others, however, suggested that prosecutors must be confident that she has compelling evidence they can use against her co-defendants.
“You don’t give a no-jail plea deal unless that person’s got something very good to say that will help your case against the others,” Chris Christie, a former federal prosecutor who is challenging Mr. Trump for the Republican presidential nomination, told a reporter in New Hampshire shortly after Ms. Powell’s deal was entered.
Until now, the only people known to have reached cooperation deals in Mr. Trump’s four criminal cases were relatively minor figures.
In September, Scott Hall, a Georgia bail bondsman charged with Mr. Trump and 17 others in the Georgia election case, pleaded guilty and agreed to testify against his co-defendants.
A few weeks earlier, the federal prosecutors who indicted Mr. Trump on charges of illegally retaining dozens of classified documents and then obstructing the government’s attempts to get them back secured the cooperation of Yuscil Taveras, one of Mr. Trump’s I.T. experts. Mr. Taveras told investigators about an effort to strong-arm him into deleting surveillance footage integral to the inquiry.
Mr. Trump likes to maintain a measure of control in everything connected to him, and the news that Ms. Powell could potentially take the stand against him was an unwelcome development. Still, he was described by people who have spoken with him as more focused on the New York attorney general’s case against him than Ms. Powell’s plea, although they conceded he was troubled by it.
Officials have long maintained that prosecutors working under Ms. Willis and those who work for the special counsel Jack Smith, who is overseeing the federal prosecutions of Mr. Trump, have not coordinated their efforts even though their separate cases involve many of the same players and cover substantially similar ground. That makes it difficult to know whether Ms. Powell might eventually reach a deal with Mr. Smith as she did with Ms. Willis.
Even if Mr. Smith were to subpoena Ms. Powell as a witness in his case, she could refuse to testify by exercising her Fifth Amendment rights against self-incrimination in Washington despite the fact that she had effectively waived those rights in Georgia. That said, any testimony Ms. Powell gives from the stand in Fulton County could in theory be used against her should Mr. Smith ultimately decide to bring charges against her.
Regardless, Ms. Powell’s new role as a cooperating witness in Georgia presents a potentially serious threat to Mr. Trump given that she is in a position to speak firsthand about a range of schemes that he and his allies undertook to subvert the democratic process. Her testimony could also lend legitimacy to the efforts to prosecute other people close to Mr. Trump, including Mr. Giuliani, who like Ms. Powell is included as an unindicted co-conspirator in the federal election case.
While Ms. Powell rushed to Mr. Trump’s aid almost as soon as he lost the presidential race, she had been on his radar for months before that. She had spoken with him privately earlier in his presidency, as she defended his first national security adviser, Michael T. Flynn, in a case stemming from the investigation into possible ties between the Trump 2016 campaign and Russians.
People close to Mr. Trump said he had liked what he perceived as her moxie.
But after Election Day in 2020, Ms. Powell became best known for filing the lawsuits accusing Dominion Voting Systems of working with a sprawling cast of characters to rig the election against Mr. Trump.
Ms. Powell also joined Mr. Giuliani at an infamous news conference in mid-November 2020. There, as dark rivulets of liquid dripped down Mr. Giuliani’s face, Ms. Powell pushed her theories about a vast conspiracy of “globalist dictators” and “corporations” that were working against Mr. Trump.
Mr. Trump, sitting with a group of aides the next day, took a call from Ms. Powell during which she reiterated those claims, according to the report by the House select committee investigating his efforts to subvert the election results. “While she was speaking, the president muted his speakerphone and laughed at Powell, telling the others in the room, ‘This does sound crazy, doesn’t it?’” according to testimony cited by the panel’s report.
Even though the Trump campaign soon distanced itself from her, saying she was “practicing law on her own,” Ms. Powell got close to Mr. Trump again within weeks. She joined Mr. Flynn and others in a wild meeting in the Oval Office where they tried to pitch Mr. Trump on using the military to take control of voting machines around the country. As part of that discussion, Ms. Powell sought to have herself named as a special counsel with extraordinary powers to investigate alleged election fraud.
Mr. Trump was seriously considering the move, even discussing getting her a security clearance, The New York Times reported at the time. Ms. Powell returned to the White House in the days after the meeting, but never got the role she had been seeking.
Just before Mr. Biden took office, Dominion filed a sprawling defamation lawsuit against her, saying that she oversaw “a viral disinformation campaign” that fed lies about the election to millions of people. Eight months later, a federal judge in Michigan imposed sanctions on her for “a historic and profound abuse of the judicial process” by filing her suits against Dominion.
In an interview for a book about his history and his presidency in April 2021, Mr. Trump denounced Ms. Powell for having said a short time before, in relation to the Dominion case, that no “reasonable” person would have believed what she was saying.
“I was very disappointed in her statement,” Mr. Trump said in the interview, adding: “Why did anyone take her seriously? That is so demeaning for her to say about herself.” He described it as “shocking.”
“She was bold and strong,” Mr. Trump said. “And then she made a statement and I said, ‘Maybe she’s not so bold or maybe she’s not so strong.’”