Originally Published: 20 MAR 23 05:03 ET

Updated: 20 MAR 23 14:54 ET

By Jack Forrest and Zachary Cohen, CNN

(CNN) -- The grand jury empaneled as part of the investigation into Donald Trump's alleged role in a scheme to pay hush money to adult film star Stormy Daniels is set to hear from a potentially key witness put forward by the former president's legal team on Monday.

Robert Costello, an attorney who has previously represented Trump allies like Steve Bannon and Rudy Giuliani, arrived at the courthouse in New York on Monday afternoon to deliver grand jury testimony at the request of the former president's legal team, he told CNN.

Costello is expected to offer evidence that contradicts testimony provided by former Trump attorney Michael Cohen, who has admitted to paying $130,000 to Daniels just before the 2016 election to stop her from going public about an alleged affair with the former president. Trump has denied the affair.

Costello's appearance is unusual for grand jury proceedings, particularly because he has previously served as Cohen's legal adviser.

The Manhattan District Attorney's Office has signaled they are closing in on a decision of whether or not to indict the former president. Trump on Saturday said he expects to be arrested this week in connection with the yearslong case and called on his supporters to protest any such move, prompting security concerns in New York City.

Cohen told MSNBC that he has been asked to be at the district attorney's office Monday as a rebuttal witness, but said he wasn't sure if he'd be answering questions before the grand jury or in an interview with the Manhattan DA. "Whether it's before the grand jury or just with them for another interview, again, I'm not sure," Cohen said.

Costello contacted both the Manhattan District Attorney's Office and lawyers for Trump to offer evidence that contradicts what Cohen has said publicly about the alleged hush money payments, a person familiar with the matter said.

CNN has reached out to the DA's office.

Costello seeks to undercut Cohen claims

Costello told CNN he has handed over hundreds of documents to the Manhattan district attorney, including more than 300 emails related to Cohen, and plans to offer evidence that contradicts what Cohen has said publicly about hush money payments he facilitated.

Costello also says he has turned over contemporaneous memos of his meeting with Cohen at the Regency hotel several years ago and his interview with the US attorney after Cohen waived attorney-client privilege.

In a letter to prosecutors obtained by CNN, Trump's legal team requested that Costello testify before the grand jury about what Cohen told him. Costello contends Cohen repeatedly told him he did not know of any criminal activity by Trump in any matter, according to the letter, which also says Cohen waived attorney-client privilege.

"Given the central role Mr. Cohen's testimony plays in this grand jury investigation, we believe this testimony -- that Mr. Cohen previously stated that he was unaware of any criminal activity by President Trump -- is crucial to allow the grand jury to exercise its 'role ... as a buffer between the accused and the government,'" Susan Necheles, who is representing Trump in the Manhattan case, wrote in the letter.

Costello played a questionable role in the Daniels case when it was being handled by federal investigators.

After federal investigators searched Cohen's home and office in 2018 but before he pleaded guilty to federal crimes, Costello told Cohen he was in touch with Rudy Giuliani -- then Trump's lawyer in the Russia probe -- and wrote, "Sleep well tonight, you have friends in high places," according to the report produced by former special counsel Robert Mueller.

The then-special counsel found that Trump and his allies at the time may have been trying to persuade Cohen not to cooperate with investigators, who were looking into the Daniels payment and other business dealings.

Investigative documents from the Mueller probe note Costello spoke to Cohen about pardons and was angry when Cohen hired a prominent New York defense attorney, according to Cohen. The Mueller investigation didn't describe Costello as Cohen's lawyer.

Costello also tried to help Bannon as his lawyer during the January 6 congressional investigation and winded up being a witness in Bannon's contempt case from whom prosecutors gathered some communications records.

Trump raises stakes with call to protest

An indictment against Trump would be historic, marking the first time a former US president or major presidential candidate has ever been criminally charged. And while Trump has an extensive history of civil litigation both before and after taking office, a criminal charge would represent a dramatic escalation of his legal woes as he works to recapture the White House.

The former president has been agitating for his team to get his base riled up and believes that an indictment would help him politically, multiple people briefed on the matter told CNN, though in private, he has called the looming development "unfair."

Publicly, he has taken to social media to try and leverage the situation. In a post on Saturday, Trump said he expects to be arrested in connection with the investigation, though he did not expand on why he thought charges were imminent and his team said after his post that it had not received any notifications from prosecutors. In an echo of Trump's appeals to supporters in the wake of the 2020 presidential election, he called for action, writing: "Protest, take our nation back."

In his most recent posts, Trump has refrained from calling for protests, as allies have told him such comments are not helpful, and instead focused on criticizing Manhattan District Attorney Alvin Bragg. In an example of the difficult situation for Republicans, House Speaker Kevin McCarthy, who has attacked Bragg and said any possible indictment would be "an outrageous abuse of power," has also said he doesn't believe people should protest.

"And I think President Trump, if you talked to him, he doesn't believe that either," the California Republican told reporters Sunday night.

But a lawyer for Trump, Alina Habba, predicted "mayhem" in New York if an indictment comes down.

"If there are (security) concerns, that's rightfully so," Habba told CNN's Paula Reid Sunday evening. "If this is what we're doing in this country, you better secure the premises. ... People are going to be upset."

She also said that "no one wants anyone to get hurt" and Trump supporters should be "peaceful" while exercising their First Amendment right to protest.

Bragg wrote in an email to staff on Saturday that his office will "not tolerate attempts to intimidate our office or threaten the rule of law in New York." He said his office is coordinating with the New York City Police Department and the court to "ensure that any specific or credible threats against the office will be fully investigated and that the proper safeguards are in place so all 1,600 of us have a secure work environment," according to the email, which was first obtained by Politico.

This story has been updated with additional developments.