One morning last month in an Atlanta federal courtroom, Mark Meadows, former President Donald J. Trump’s final chief of staff, was in the hot seat. And he was the one who had put himself there.

Mr. Meadows’s lawyer had made the surprise move of asking his client to testify in an effort to have the Georgia election interference case against him moved to federal court, a venue where his chances of acquittal, or even having the case thrown out, may be a bit better than in the state court where he has been charged.

At issue was whether his actions — as described in the indictment of Mr. Trump, Mr. Meadows and 17 others in Fulton County, Ga., last month — could be considered within the scope of his duties as White House chief of staff. Mr. Meadows made the case for that under questioning by his lawyer, but he hit a snag when a prosecutor asked whether he had “any role” in coordinating the bogus electors who were used in a last-ditch effort to keep Mr. Trump in power after he lost the 2020 election.

“No, I did not,” he replied.

The prosecutor then introduced into the record a December 2020 email that Mr. Meadows wrote to a Trump campaign staff member. In it, Mr. Meadows wrote, “We just need to have someone coordinating the electors for the states.”

The exchange, which prosecutors will almost certainly use against Mr. Meadows at trial, underscored the high-stakes gamble that he took by testifying. So far, the gamble has not paid off: In early September, U.S. District Judge Steve C. Jones declined to move his case to federal court.

Mr. Meadows has appealed. But his testimony may have given ammunition to Georgia prosecutors as they prepare to try him, Mr. Trump and the 17 other defendants. Legal experts say that Mr. Meadows may have damaged his credibility while weakening his claim to immunity from state prosecution as a federal official, given his struggles to articulate how the actions ascribed to him in the indictment were part of his official duties rather than in service of the Trump campaign.

“He did do a number of things which will make the prosecutors’ job easier,” said Caren Morrison, a former federal prosecutor and associate law professor at Georgia State University, of Mr. Meadows’s testimony. “He’s created some additional trouble for himself, I think, as well as for Trump.”

With an affable demeanor, Mr. Meadows has been described as a less than assertive chief of staff by former White House aides, and as someone with a habit of telling people what they wanted to hear.

During the hearing in Atlanta, Mr. Meadows portrayed himself as a supremely busy man whose White House job often put him at the nexus of policy and politics. While acknowledging that he interacted with the Trump campaign for various reasons, such as working out scheduling issues, he emphasized that he operated independently of it.

But Anna Cross, the prosecutor who cross-examined him, stressed that Mr. Meadows had specifically offered financial assistance from the Trump campaign in order to help with ballot verification in Georgia, in a December 2020 email he sent to an official there. Why, she asked, had Mr. Meadows made such an offer “on behalf of the Trump campaign?”

Mr. Meadows said he was just trying to “speed things up,” and he explained that it was virtually impossible to keep politics out of the White House chief of staff job. That may ultimately prove convincing to a jury. The role is unquestionably unique; former President Gerald Ford described the chief of staff as a “filter” and “alter ego” for a president, whose own job involves mixing the duties of governing and campaigning.

Mr. Meadows’s lawyer, George J. Terwilliger III, declined to comment. In court filings, the Meadows legal team argues that he did nothing illegal but was rather fulfilling his duties as the president’s right-hand man.

Putting Mr. Meadows on the stand at this stage may even have offered certain advantages for his lawyers. “You get a little bit of a review of where the state’s going to go, and his lawyer gets a preview of how he’s going to do on the stand,” said Noah H. Pines, an Atlanta-area criminal defense lawyer and former prosecutor.

At the same time, Mr. Pines said, every word of Mr. Meadows’s testimony may now be used against him at trial.

Mr. Meadows said he went to Cobb County, Ga., in December 2020 to observe an audit of signatures on ballots — one of the acts laid out in the indictment — because he was in the Atlanta area visiting his children for Christmas. He anticipated Mr. Trump having questions about the procedure, he said, and so he visited in the spirit of “checking off a box to say this has been checked, that’s a question that’s been asked and answered.”

Mr. Meadows also tried to explain his role in the now-famous Jan. 2, 2021, phone call from Mr. Trump to Brad Raffensperger, Georgia’s secretary of state. During that call, Mr. Trump told Mr. Raffensperger he needed to “find” 11,780 votes, one more than what he needed to win in Georgia.

Mr. Meadows, who set up the call, also spoke on it, saying he hoped the president’s team could “get the access to the secretary of state’s data” to investigate claims of voter fraud that Mr. Trump was pushing.

The phone call to Mr. Raffensperger, he said at the recent hearing, was part of his effort to move closer to a “peaceful transfer of power” by taking some of Mr. Trump’s lingering concerns about voter fraud in Georgia “off of the president’s concern list.”

“It seemed like his claim was, ‘My job was to do anything and everything required by the president,’” said Melissa D. Redmon, a law professor at the University of Georgia who has been an Atlanta prosecutor. “The problem is that plays into the state’s theory that some of the things that you did on behalf of the president were prohibited by your office, as an employee of the White House.”

Ms. Morrison also said that it could prove damaging for Mr. Trump’s case that Mr. Meadows acknowledged in testimony that Mr. Trump had a “personal interest” in seeking a reversal of the election results in Georgia.

“That’s partially making the argument that the prosecution wants to make, that Trump being all up in Georgia’s business about the election was not part of his executive duties as president,” she said.

The Meadows legal team will have another chance to make its case for moving the case to federal court, this time to the United States Court of Appeals for the 11th Circuit. While he waits, four other defendants seeking to move their cases to federal court will get hearings this week, starting with Jeffrey Clark, a former Justice Department official, on Monday.

It remains unclear whether any of them will take the stand.