Even as Donald Trump prepares to dial up his campaign to take back the White House, the former US president’s political and personal fate may already have been decided by the secret workings of a grand jury in Georgia.

The 23-member panel, convened to consider whether Trump and others committed crimes in trying to overturn his defeat in Georgia when it appeared the state might decide the outcome of the entire 2020 presidential election, was dissolved on Monday after submitting its conclusions and asking that they be made public.

If the grand jury’s report recommends prosecution, a county district attorney in Atlanta, Fani Willis, will face the most consequential decision of her career – whether, for the first time in American history, to charge a former president with a criminal offence.

That could result in Trump sitting behind bars in Georgia when he expects to be out on the campaign trail. Provided he is not already serving time as the result of a federal investigation into his attempts to pressure election officials in several other states to rig the vote and his part in the 6 January 2021 storming of the Capitol.

A judge has scheduled a hearing later this month to consider arguments over whether the grand jury’s report should be made public while Willis, the Fulton county district attorney, scrutinises its findings.

In November, the day before Trump announced he was again running for the White House, the Brookings Institution in Washington published a report that concluded he is “at substantial risk of prosecution” in Georgia including for improperly influencing government officials, forgery and criminal solicitation. The report said Trump may even be vulnerable to charges under anti-racketeering laws written to combat the mafia.

Norman Eisen, the lead author of the Brookings report and former White House special counsel for ethics and government reform, said he thinks charges against Trump are “highly likely”.

“The evidence is powerful and the law is very favourable to the prosecutors in Georgia,” he said. “I believe the [special grand jury] report very likely calls for the prosecution of Trump and his co-conspirators.”

Eisen said that the federal case is not as far along but that the congressional committee investigating the events of January 6 laid out a “powerful case” for charges against Trump.

He said that the prosecution of a former president would be “momentous”.

“But, of course, so was Trump’s decision to lead an attempted coup. That was momentous in a very negative way. This is momentous as a defence of the rule of law and American democracy,” said Eisen.

Georgia prosecutors have warned at least 18 other people that they are targets of the investigation and could be charged, including Trump’s close ally and lawyer, the former New York City mayor Rudolph Giuliani who has, among other things, been accused of spreading conspiracy theories in testimony to the Georgia legislature.

Willis launched her investigation into “a multistate, coordinated plan by the Trump campaign to influence the results” just weeks after the former president left office. The investigation initially focused on a tape recording of Trump pressuring Georgia’s secretary of state, Brad Raffensperger, to conjure nearly 12,000 votes out of thin air in order to overturn Joe Biden’s win.

Willis expanded the investigation as more evidence emerged of Trump and his allies attempting to manipulate the results, including the appointment of a sham slate of 16 electors to replace the state’s legitimate members of the electoral college. The fake electors included the chair of the Georgia Republican party, David Shafer, and Republican members of the state legislature who have been warned that they are at risk of prosecution.

The Fulton county district attorney has told state officials that her office is investigating an array of crimes against Trump and others, including criminal solicitation to commit election fraud, intentional interference with the performance of election duties, conspiracy and racketeering. Convictions potentially carry significant prison sentences.

Fulton superior court approved the appointment of the special grand jury last year at Willis’s request. She reflected on the consequence of investigating a former president as the jurors began their work.

“I don’t want you to think I’m naive or I don’t get the gravity of the situation,” Willis told the Atlanta Journal-Constitution. “I get the gravity of it … But it’s just like every other case. You just have to do your due diligence.”

Special grand juries are rare in Georgia. Unlike the regular kind, they cannot indict. But they can sit for much longer and have wider powers to subpoena. Willis recognised that if she was to build a case against such a divisive political figure as Trump, and convince a jury in a criminal trial, the evidence would have to be rock solid, and that would take time and depth.

Willis used the grand jury’s powers to good effect. She called a parade of witness, including many of Trump’s closest allies and lawyers. Some fought their subpoenas including Senator Lindsey Graham who went all the way to the US supreme court in a failed attempt to avoid giving evidence.

The star witness was Raffensperger, a Republican who voted for Trump and oversaw his state’s elections. When the numbers stacked up against the president in Georgia, Trump knew where to turn.

Raffensperger spoke to the special grand jury for several hours in June. Georgia’s secretary of state has not commented publicly about his testimony but in his book, Integrity Counts, Raffensperger recounts receiving a call from Trump as he sat in his kitchen with his wife, Tricia, on 2 January 2021. He put the president on speakerphone.

Raffensperger had an idea what to expect. Trump had already “tweeted insults and threats at me and Georgia governor Brian Kemp”. For an hour, the president tried to persuade Raffensperger to overturn the vote.

“So, we’ve spent a lot of time on this and if we could just go over some of the numbers, I think it’s pretty clear that we won. We won very substantially in Georgia,” Trump said on the call.

Raffensperger said he was tempted to interrupt and disagree but did not out of respect.

Trump went on: “I just want to find 11,780 votes … because we won the state.”

Raffensperger told the president he “could not do that because the data did not support it”.

Trump tried to claim that the vote had been rigged by alleging that ballot boxes were stuffed and other irregularities. Then the president said: “All of this stuff is very dangerous stuff when you talk about no criminality. I think it’s very dangerous for you to say that.”

Raffensperger saw that for what it was.

“I felt then – and still believe today – that this was a threat,” he wrote. “Others obviously thought so, too, because some of Trump’s more radical followers have responded as if it was their duty to carry out this threat.”

Raffensperger said he and his wife were subject to death threats.

Willis had more than the witness’s word for it. Raffensperger recorded the call, providing powerful and indisputable evidence.

The Fulton county district attorney brought a parade of other witnesses before the grand jury including the then White House chief of staff, Mark Meadows, and Graham, who placed calls to Raffensperger to suggest he should throw out some absentee ballots.

Giuliani is likely to have been asked about false testimony he gave to Georgia legislators the month after the presidential election, including claims that voting machines were rigged and that thousands of teenagers below the voting age had cast ballots. A New York court suspended his licence to practice law last year over his “demonstrably false and misleading statements regarding the Georgia presidential election results”.

Willis has also gathered evidence about attempts to pressure a Fulton county poll worker and her daughter to wrongly say they committed election fraud by ballot stuffing, the sudden resignation of a US attorney in Atlanta under pressure from Trump officials to more aggressively investigate alleged election fraud, and of an IT services company hired by one of Trump’s lawyers that illegally copied confidential voter data from voting machines.

Those who have worked with Willis say she is unlikely to shy from prosecuting Trump if she deems it appropriate. She is known to be a fan of anti-racketeering laws, having used them to prosecute public school teachers who were part of a cheating scandal.

If Willis decides to press ahead with the case, she will need to convene a regular grand jury which has the authority to hand down indictments.

Trump has dismissed the threat to his freedom with his usual bluster. He described his conversation with Raffensperger as “perfect” and the hearings as a “witch-hunt”. He has called Willis’s investigation a “political prosecution” and “racist”, presumably because she is Black.