NEW YORK (NewsNation) — Potentially game-changing testimony before the Manhattan grand jury is expected Monday in the hush money case against former President Donald Trump.

Robert Costello, who had advised Michael Cohen — the case’s “star witness” — before the two had a falling out, is prepared to offer testimony to the grand jury, attacking the credibility of Cohen’s statements, according to a source who insisted on anonymity to discuss secret legal proceedings.

Costello had contacted a lawyer for Trump saying that he had information that contradicted Cohen’s current statements and that could be exculpatory for Trump, the person said. 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 provide testimony on Monday afternoon.

The Manhattan District Attorney’s Office declined to comment on Sunday. Prosecutors have not said when they expect to conclude their work or when or if Trump might be charged. 

However, on Saturday, Trump blasted to the world on his social media platform, Truth Social, with no proof that New York officials intended to arrest him on Tuesday.

The arrest would be in regard to an alleged $130,000 “hush money” payment made through Cohen, the former president’s attorney at the time, to porn star Stormy Daniels.

If needed and asked to do so, Cohen said he will be there as well to offer rebuttal testimony.

Trump took aim at Cohen Sunday night, calling him a “convicted felon, jailbird and serial fake storyteller and liar.” He described the whole investigation as a “witch hunt.” Trump’s attorney also claimed Sunday night that the investigation was not about justice.

“This is a politically motivated prosecution. This is the thing that’s happening in this country now, where we’re using the justice system as a weapon. It’s the weaponization of a prosecutor’s office to get a political opponent,” Joe Tacopina said.

Experts said they expect the top count against the former president to be falsifying business records in the first degree.

Former New Jersey Superior Court Judge Andrew Napolitano explained that the government will have to satisfy a jury beyond a reasonable doubt that the $130,000 that Cohen took out of his own pocket to pay Daniels, and for which he was then eventually slowly reimbursed, was from the Trump Organization. And then that it was a circuitous route to avoid the laws which prevent corporations from making donations to a campaign. 

He continued, saying that Trump’s people will argue that Daniels was paid off not to help the campaign, but to help Trump’s marriage.

“It’s not an easy case to prove. It is not the strongest case against Donald Trump. But it is a case because the laws are there and the government believes it can prove that Trump personally orchestrated the violation of these campaign finance laws,” Napolitano said.

District Attorney Alvin Bragg, who took office last year, is leading the charge against Trump. Elected as a Democrat, he boasted time and time again on the campaign trail that he sued Trump over 100 times.

Following Trump’s calls for protests over the looming indictment, Bragg sent out this internal note, stating, “We do not tolerate attempts to intimidate our office or threaten the rule of law in New York … as with all of our investigations, we will continue to apply the law evenly and fairly.”

Trump continues to insist that no crime was committed. Still, there are many questions about what a possible indictment of a former president would look like. Many legal experts said not to expect any fireworks.

“Former President Trump is not going to be dragged in front of a scrum of reporters,” Danya Perry, the former assistant U.S. attorney for the southern district of New York, said. “There’s not gonna be a perp walk, he won’t be put in handcuffs or shackles. He will self-surrender.”

But even an indictment could not prohibit Trump from continuing a run to regain the presidency, former Deputy Assistant Attorney General Tom Dupree said during an interview on “NewsNation Live.”

“There is nothing in our law or constitution that would prohibit former President Trump from continuing his campaign to regain the presidency, even if he were indicted,” Dupree said.

Dupree said it’s uncharted territory, explaining Americans have never seen anything like this in history.