As Donald Trump's hush money trial in Manhattan nears its end, experts say criminal defense attorney Robert Costello, who once advised the former president's ex-fixer and key prosecution witness Michael Cohen, has surfaced as a "likely" and potentially bombshell witness for the defense.

Trump's legal team repeatedly attacked Cohen's character and the reliability of his testimony while cross-examining the disbarred lawyer, convicted felon and admitted perjurer on Tuesday and Thursday, before the trial adjourned for a long weekend.

Costello, a former Manhattan federal prosecutor who has contradicted Cohen's key testimony against Trump in public statements and under oath, could further undermine Cohen's credibility in front of the jury and bolster Trump's defense.

Trump's attorneys have left open the possibility of calling witnesses, including Trump, though experts say it is unlikely that he will testify.

Costello told Law360 Friday that Trump's lead defense attorney, Todd Blanche, called him about three weeks ago, but he's had no other contact with the defense team since then. When asked if he believed he would be called to the stand, Costello said he couldn't speak for Trump's defense team, though he added that he was ready to testify if needed.

"This is a subject of which I am well acquainted," he said. "I don't know how much preparation I need. I don't think I need any, but that's just me."

Trump is charged with 34 felony counts of falsifying business records related to $130,000 in hush money payments to porn star Stormy Daniels as part of a scheme to bury negative stories during his 2016 presidential campaign. Daniels was planning to go public with allegations she had a sexual encounter with Trump, and he wanted to keep her quiet before the election, according to prosecutors.

Cohen, who claims he paid Daniels at Trump's behest, pled guilty in 2018 to federal campaign finance law violations stemming from the alleged hush money scheme. He also pled guilty to federal charges of tax evasion and making false statements to a bank, and he's admitted to lying to Congress.

Last year, Trump's attorneys requested that Costello appear before the grand jury in Trump's hush money case, a "very unusual move" that suggests he'll be called as a defense witness, Nina Marino, a white collar defense lawyer at Los Angeles-based Kaplan Marino PC, told Law360.

"They put him in front of the grand jury because he has favorable testimony for the defense," she said. "There's always risks in calling a witness, but if I were defending this case, I would call Costello. He could really impugn Cohen's credibility. And Cohen is the anchor to the state's case."

Trump's jury has already heard about Costello through Cohen's direct examination. He testified Tuesday that Costello was "incredibly close" with ex-Trump attorney Rudy Giuliani and had boasted about having a "backchannel" of communication with Trump.

Costello formerly represented Giuliani. Last year, Costello and his law firm sued Giuliani over more than $1.3 million in unpaid legal bills.

"The way the prosecution brought out Costello in their direct case — that he was close to Giuliani and therefore close to Trump, so Costello would have a motive to come in and say something favorable to Trump — showed that they were concerned about what he might say," said Jon Sale, co-chair of the white collar practice at Nelson Mullins Riley & Scarborough LLP.

"Costello is a likely witness," he said.

Whether Trump's team calls Costello to testify depends, in part, on how strong of a case they believe the government has presented to the jury, noted former federal prosecutor Josh Robbins, co-chair of Buchalter PC's white collar practice.

"The government has done a pretty good job of lightening the load Cohen has to carry," he said, "by showing through other evidence Trump's motive to cover up the affair and the hush money payments, as well as his modus operandi of tightly controlling expenditures."

Costello has said publicly that he's not been subpoenaed to testify at Trump's trial, but he has indicated that he is willing to take the stand in Trump's defense. He testified Wednesday during a Congressional hearing that Cohen's key testimony in Trump's trial was false.

He told the House Select Subcommittee on the Weaponization of the Federal Government that Cohen revealed to him, before Trump was charged in the hush money case, that he acted alone when he paid Daniels and was seeking to protect Trump's wife, rather than his presidential campaign.

"Michael Cohen said, 'I didn't believe the allegation. But I knew that such an allegation would be terribly embarrassing,'" Costello told House lawmakers. "He said, 'I didn't want to embarrass Melania Trump.' He said, 'That's why I decided to take care of this on my own.'"

In reaction to Costello's Congressional testimony, Rep. Dan Goldman, D-N.Y., accused Costello of "jury tampering."

Cohen, meanwhile, testified Monday in a Manhattan courtroom that Trump directed him to pay off Daniels and, crucially, had agreed to classify his reimbursements for the hush money payments as "legal services."

To secure a conviction, prosecutors must prove Trump not only tried to conceal the alleged payments to Daniels, but did so with the intent to cover up or commit another crime, such as state or federal election law violations.

Costello's statements directly contradicting Cohen's testimony, which serves as an integral part of the prosecution's case, could be "dynamite" for Trump's defense, according to Sale.

However, calling Costello as a witness also poses risks for Trump. The move would open the door for prosecutors to cross-examine Costello about his communications with Cohen, among other things, while also asserting that he is a Trump ally.

"What we do not know is what Costello was asked before the grand jury," said former New York prosecutor Marc Scholl of Lewis Baach Kaufmann Middlemiss PLLC. "Also, if Costello is the only defense witness called, then the prosecution is free to comment on weaknesses in the defense — something that the prosecution cannot really do if no witnesses are called."

Scholl added, "Tactically, it could well be better for the defense to present no witnesses. If no witness is presented, then the whole argument to the jury revolves around the adequacy of what the prosecution presented."

Cohen is set to retake the stand Monday, when his cross-examination will continue. He is expected to be the prosecution's final witness.

The government is represented by Joshua Steinglass, Christopher Conroy, Susan Hoffinger, Matthew Colangelo, Rebecca Mangold and Katherine Ellis of the New York County District Attorney's Office.

Trump is represented by Todd Blanche, Emil Bove and Stephen Weiss of Blanche Law, Susan Necheles and Gedalia Stern of NechelesLaw LLP and Steven Yurowitz of Yurowitz Law PLLC.

The case is New York v. Trump, case number 71543-23, in the Supreme Court of the State of New York, County of New York.

--Additional reporting by Frank G. Runyeon and Stewart Bishop. Editing by Alex Hubbard.