An indictment could also test a Republican Party already divided over whether to support Trump next year, in part due to his efforts to undermine his 2020 election loss.

Trump denies any wrongdoing and has slammed the Manhattan district attorney’s office probe as politically motivated.

What’s the probe about?

The grand jury has been probing Trump’s involvement in a US$130,000 payment made in 2016 to the porn actress Stormy Daniels to keep her from going public about a sexual encounter she said she had with him years earlier.

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Trump’s lawyer Michael Cohen paid Daniels, whose real name is Stephanie Clifford, through a shell company before being reimbursed by Trump, whose company, the Trump Organisation, logged the reimbursements as legal expenses.

Earlier in 2016, Cohen also arranged for former Playboy model Karen McDougal to be paid US$150,000 by the publisher of the supermarket tabloid The National Enquirer, which then squelched her story in a journalistically dubious practice known as “catch-and-kill”.

Trump denies having sex with either woman.

Trump’s company “grossed up” Cohen’s reimbursement for the Daniels payment to defray tax payments, according to federal prosecutors who filed criminal charges against the lawyer in connection with the payments in 2018. In all, Cohen got US$360,000 plus a US$60,000 bonus, for a total of US$420,000.

Cohen pleaded guilty to violating federal campaign finance law in connection with the payments. Federal prosecutors say the payments amounted to illegal, unreported assistance to Trump’s campaign. But they declined to file charges against Trump himself.

Manhattan District Attorney Alvin Bragg’s team appears to be looking at whether Trump or anyone committed crimes in New York state in arranging the payments, or in the way they accounted for them internally in the Trump Organisation.

But can a former president be indicted?

In a word, yes. Long-standing Justice Department policy prohibits the federal indictment of a sitting president, but Trump, two years out of office, no longer enjoys that legal shield. And the New York case is not a federal probe anyway.

What is this grand jury?

A grand jury is made up of people drawn from the community, similar to a trial jury. But unlike juries that hear trials, grand juries don’t decide whether someone is guilty or innocent. They only decide whether there is sufficient evidence for someone to be charged. Grand juries exist in the federal court system and in many states.

Proceedings are closed to the public, including the media. There is no judge present nor anyone representing the accused.

Prosecutors call and question witnesses, and grand jurors can also ask questions. In New York, the person who could be indicted may ask for a certain witness, though it’s up to grand jurors.

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New York grand juries have 23 people. At least 16 must be present to hear evidence or deliberate. Twelve have to agree there is enough evidence in order to issue an indictment. The grand jury may also find there is not enough evidence of a crime or direct the prosecutor to file lesser charges.

Centuries-old rules have kept grand juries under wraps to protect the reputations of people who end up not being charged, to encourage reluctant witnesses to testify, to prevent those about to be indicted from fleeing and to guard against outside pressure.

Grand juries have long been criticised as little more than rubber stamps for prosecutors. Former New York judge Sol Wachtler famously said that prosecutors could convince a grand jury to “indict a ham sandwich”. Defenders of the process say it is a crucial safeguard against politically motivated prosecutions.

Who has testified in this case

One of the final witnesses being called was Robert Costello, who was once a legal adviser to Cohen, the government’s key witness in the investigation.

The men have since had a falling out, and Costello has indicated that he has information he believes would undercut the credibility of Cohen and contradict his current incriminating statements about Trump.

Costello contacted a lawyer for Trump saying he had information that could be exculpatory for Trump, according to a person familiar with the matter who insisted on anonymity to discuss secret legal proceedings. The lawyer brought it to the attention of the district attorney’s office, which last week subpoenaed Costello’s law firm for records and invited him to testify.

He was at the building where the jurors were meeting on Monday, invited by prosecutors, ensuring the grand jury had an opportunity to consider testimony or evidence that could weaken the case for indicting.

Trump was also invited to testify, but his lawyer has said the former president has no plans to participate.

What about the political ramifications?

Trump says charges would actually help him in the 2024 presidential contest. Long-time ally Lindsey Graham, senator from South Carolina, said Saturday that District Attorney Bragg “has done more to help Donald Trump get elected”.

Florida Governor Ron DeSantis, considering joining the Republican field, criticises the Trump investigation as politically motivated, “fundamentally wrong”. But he also threw one of his first jabs at the former president in a quip likely to intensify their rivalry.

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DeSantis said he personally doesn’t “know what goes into paying hush money to a porn star to secure silence over some kind of alleged affair”.

Comments by other potential rivals, eager to convince voters it is time to move on from the former president but also contending with the fact that he remains the most popular figure in the party:

– During a Saturday visit to Iowa, former vice-president Mike Pence called the idea of indicting a former president “deeply troubling”.

– Asa Hutchinson, the former Arkansas governor mulling his own 2024 bid, said he didn’t expect Trump to withdraw from the race after an indictment, though that would be the “right” thing to do.

– Former South Carolina governor Nikki Haley, a declared candidate who also served as Trump’s UN ambassador, said Monday on Fox News that Bragg’s case was an attempt at scoring “political points”, adding: “You never want to condone any sort of prosecution that’s being politicised”.

“At the end of the day, not one single person’s opinion of him will be any different after indictment than it was before,” veteran Republican Party operative Terry Sullivan said in an interview. “All of his perceived negatives are already baked into his name ID with voters.”

What about other Trump investigations?

The New York probe is among many legal woes Trump is facing.

The Justice Department is investigating his retention of top secret government documents at his Florida estate, Mar-a-Lago, after leaving the White House, as well as possible efforts to obstruct that probe. Federal investigators are also still probing the January 6, 2021, insurrection and efforts to overturn the election Trump falsely claimed was stolen.

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Portions of a report from a special grand jury in Georgia that investigated whether Trump and his allies illegally interfered in the 2020 election in Georgia shows jurors believed “one or more witnesses” committed perjury and urged local prosecutors to bring charges. The former president never testified, but the report didn’t foreclose the possibility of other charges.

What’s next in New York?

It’s unclear. Trump declared in a social media post over the weekend that he expects to be taken into custody on Tuesday and urged supporters to protest his possible arrest. However, there has been no public announcement of any time frame for the grand jury’s secret work. A Trump spokesperson said there has been no notification from Bragg’s office.

Law enforcement officials have been making security preparations for the possibility of an indictment in coming days or weeks – or a court appearance by the president himself.