Workers erected barricades around a Manhattan courthouse on Monday as New York City braced for a possible indictment of Donald Trump over an alleged hush-money payment to porn star Stormy Daniels during his 2016 campaign.

It would be the first-ever criminal case against any US president. On Saturday, Trump urged followers on social media to protest what he said was his looming arrest.

In his call for protests, Trump raised concerns for law enforcement that supporters might engage in violence similar to the Jan 6, 2021, attack on the US Capitol in Washington.

Fearing a trap, however, several far-right grassroots groups have opted not to heed his call, security analysts said.

A grand jury, which heard further testimony on Monday, could bring charges as soon as this week. Trump, who is seeking the Republican nomination for the White House again in 2024, had predicted he would be arrested today.

On Monday, the grand jury heard from a witness, lawyer Robert Costello, who said Trump's former fixer Michael Cohen had handled the hush-money payments without Trump's involvement.

"Michael Cohen decided on his own - that's what he told us - on his own, to see if he could take care of this," Costello told reporters after testifying to the grand jury at Trump's lawyers' request.

Cohen, who testified twice before the grand jury, has said publicly Trump directed him to make the payments on Trump's behalf.

An indictment could hurt Trump's comeback attempt. Some 44 percent of Republicans say he should drop out of the presidential race if he is indicted, according to a seven-day Reuters/Ipsos poll that concluded on Monday.

The investigation by Manhattan district attorney Alvin Bragg is one of several legal challenges facing Trump. His office did not immediately respond to a request for comment.

Cohen pleaded guilty in 2018 to federal campaign finance violations tied to his arranging payments to Daniels, whose legal name is Stephanie Clifford, and another woman in exchange for their silence about affairs they claimed with Trump.

Trump has denied that any such affairs took place.

The Manhattan District Attorney's office had asked that Cohen be available as a rebuttal witness, but he was told on Monday afternoon that his testimony was not needed, according to his lawyer Lanny Davis. Cohen told MSNBC he had not been asked to return on Wednesday.

No sign of unrest

New York Mayor Eric Adams told reporters police were monitoring social media and keeping an eye out for "inappropriate actions" in the city. The New York Police Department said there were no known credible threats.

If charged, Trump would likely have to travel from his Florida home for fingerprinting and other processing. Law enforcement officials met on Monday to discuss the logistics, several media outlets reported.

Sources have said Bragg's office was presenting evidence to a grand jury about a US$130,000 payment made to Daniels in the final weeks of the 2016 campaign.

Trump's fellow Republicans have widely criticized the probe as politically motivated.

Florida governor Ron DeSantis, Trump's rival for the Republican presidential nomination, said on Monday Bragg was imposing a "political agenda" that compromised the rule of law, but he also took a veiled swipe at Trump.

"I don't know what goes into paying hush money to a porn star to secure silence over some type of alleged affair," he told reporters.

Republicans in the US House of Representatives launched an investigation of Bragg's office with a letter seeking communications, documents and testimony related to the probe.

Trump and other Republicans have also said the Manhattan District Attorney's office should focus more on tackling crime.

Asked to comment on the letter, a spokesperson for the DA's office, citing statistics that homicides and shootings were down this year, said: "We will not be intimidated by attempts to undermine the justice process, nor will we let baseless accusations deter us from fairly applying the law."

Trump was impeached twice by the House during his presidency, once in 2019 over his conduct regarding Ukraine and again in 2021 over the attack on the US Capitol by his supporters. He was acquitted by the Senate both times.

Several more legal challenges remain

Bragg won a conviction last December against Trump's business on tax fraud charges.

But legal analysts say the hush-money case may be more difficult. Bragg's office will have to prove that Trump intended to commit a crime, and his lawyers will likely employ a range of counterattacks to try to get the case dismissed, experts say.

Trump, meanwhile, has to contend with other legal challenges, raising the possibility he will have to shuttle between campaign stops and courtrooms before the November 2024 election.

Trump's lawyers on Monday asked a Georgia court to quash a special grand jury report detailing its investigation into his alleged efforts to overturn his 2020 statewide election defeat.

The filing in Fulton County Superior Court also seeks to have the county district attorney, Fani Willis, recused from the case, arguing her media appearances and social media posts demonstrated bias against Trump.

Trump is also seeking to delay a civil fraud trial, scheduled for Oct 2, brought by the New York attorney general that alleges a decade-long scheme to manipulate the value of his assets to win better terms from bankers and insurers.

Trump faces two civil trials involving former magazine columnist E. Jean Carroll, who claims that Trump defamed her by denying he raped her. A federal judge on Monday denied a request from both sides to combine the two cases into one.

- Reuters