Donald Trump was a norm-shattering American president during four tumultuous years in the White House. Now, with a Manhattan grand jury handing down a criminal indictment against him on Thursday evening, Trump and the US have once again hurtled into uncharted — possibly dangerous — territory.

The news of the grand jury’s vote upended recent speculation that Alvin Bragg, a first-term Manhattan district attorney, might be wobbling in his commitment to a divisive case launched four years ago by his predecessor, Cyrus Vance.

It also managed to jolt a nation seemingly inured to Trump’s chaos after accusations of Russian interference in the 2016 election, two impeachments, a refusal to participate in a peaceful transfer of power, and even a deadly insurrection.

Partisans pushed aside the springtime rite of the opening day of the baseball season and rushed to social media and cable television to rehash familiar talking points: the indictment was an example of justice finally being served — or an instance of a politicised justice system used to relentlessly harass a single man. Trump endorsed the latter.

“This is political persecution and election interference at the highest level in history,” he wrote in a statement shortly after his lawyers were informed of the indictment. “Even before I was sworn in as your president . . . the radical Left Democrats — the enemy of the hard-working men and women of this Country — have been engaged in a witch-hunt.”

Lindsey Graham, the South Carolina senator and Trump ally, warned Fox News that the indictment would “destroy America”. Of Bragg he said: “He’s opened Pandora’s box against the presidency itself.”

Political analysts speculated about how the charges would impact Trump’s unique bond with Republican voters while historians invoked Nixon as they debated what it meant for the office of the presidency.

A singular reaction came from Yusef Salaam, a member of the Central Park Five, a group of young black men who were wrongly imprisoned for the 1989 rape of a white woman jogger. At the time, Trump had seized on public appetite for vengeance by taking out a full-page newspaper ad calling for the death penalty. “Karma,” Salaam, now a candidate for city council, said in a statement.

However it turns out, the indictment will permanently entwine Trump with Bragg, a man who is in some ways his perfect foil: a black, 49-year-old Harvard Law School graduate known for an even temper and a habit of eschewing the spotlight.

Even before Thursday, Bragg was becoming a hate figure among conservatives. Since taking office a little more than a year ago he has pushed to reduce incarceration and said his office would no longer prosecute many nonviolent offences. While doing so, he was pursuing Trump for a $130,000 payment to an adult film star who claimed to have had a liaison with him in 2006, which Trump denies.

Michael Cohen, who as Trump’s lawyer and fixer once said he would “take a bullet” for his boss, instead turned on him. He told the grand jury that he had made the payment on the then-candidate’s behalf just before the 2016 election to buy Daniels’ silence.

The news of the indictment has only heightened the suspense surrounding the case as the nation waits for it to be unsealed by the court so that citizens can evaluate the charges for themselves. That could happen on Friday or take several days.

Then there will be the drama of Trump’s eventual surrender to law enforcement in New York City. It is expected to take place on Tuesday, when he may become the first former American president to endure a mug shot, fingerprinting and other indignities of the criminal justice system.

“At the end of the day, he may be the former president but as of five o’clock this afternoon, he’s an indicted defendant,” said Dan Horwitz, a former prosecutor who leads the white-collar defence practice at McLaughlin Stern.

Yet Trump will require special attention. The city, for example, has maintained a heavy police presence near the courts in lower Manhattan after Trump predicted his arrest and urged his supporters to protest, arousing fears of a repeat of January 6. Yet, as Horwitz noted, “to a large degree, he’s going to be treated like anyone else”.

One offence the DA had been pursuing, according to several people briefed on the matter, focuses on the way that Trump reimbursed Cohen for the payments. According to Cohen, they were bogusly recorded in company records as legal expenses.

Falsifying business records is a misdemeanour — that is a less serious crime — under New York state law. It rises to the level of a felony carrying a possible four-year prison term if the falsification was committed to hide another crime. In this case, that crime could be a campaign finance violation because the pay-off would have aided Trump’s White House bid.

The wrinkle, though, is that the possible campaign finance violation would be a matter of federal law. It is not clear to lawyers if a New York state law and federal law can be joined in such a way. One former DA staffer called it “a bank shot” — and one that has not previously been attempted.

Karen Friedman Agnifilo, who spent the best part of three decades at the Manhattan DA’s office, agreed that it was “legally untested” but an “important case to bring” nonetheless.

For sceptics, an obvious comparison is the case of John Edwards. The onetime Democratic presidential candidate was arrested in 2011 after it emerged he had steered nearly $1mn from campaign donors to a mistress pregnant with his unborn child. The payments were made during Edwards’ 2008 campaign, and allowed the woman to go into hiding.

After denying any involvement in the payments, Edwards eventually said they were not for political purposes but to protect his dying wife. The jury acquitted him on one charge and was deadlocked on five others.

Some defence lawyers believe Trump could mount a similar argument. Meanwhile, Cohen, a convicted felon with a record of perjury, would make for a problematic witness, to say the least.

It is possible Cohen’s testimony could be bolstered by that of Allen Weisselberg, the former longtime chief financial officer of the Trump Organization. The Manhattan DA is still weighing criminal charges against Weisselberg over the company’s long-running practice of inflating the values of its properties in order to secure benefits such as bank loans or insurance coverage on better terms, according to several people familiar with the matter.

Weisselberg is serving a five-month prison sentence after pleading guilty to tax fraud. Prosecutors have not given up on what has, so far, been a frustrating attempt to persuade him to co-operate against Trump.

Even leaving those machinations aside, other legal experts believed Bragg was bound to bring a case after uncovering wrongdoing — whatever the consequences. “It feels like a real offence to me,” said Celeste Koeleveld, a partner at Clifford Chance, who called the record-keeping law a legitimate tool that state prosecutors use to keep businesses honest.

Of the political ramifications, Koeleveld said: “You don’t get to do what allegedly happened here and run roughshod over the law because you’re the president.”

Additional reporting by Joe Miller