After the prosecution rested its case against Donald J. Trump on Monday, the defense began a simple mission: destroy the credibility of the prosecution’s star witness.
And so the defense called Robert J. Costello, a lawyer who once advised Michael D. Cohen, Mr. Trump’s former fixer and the man at the heart of the prosecution’s case. They had a falling out years ago, and Mr. Costello is expected to cast Mr. Cohen as a liar.
In 2018, Mr. Costello acted as Mr. Cohen’s back channel to Mr. Trump’s legal team. This role — which came by dint of Mr. Costello’s close ties to Mr. Trump’s personal lawyer at the time, Rudolph W. Giuliani — was mostly informal. Mr. Cohen never officially retained Mr. Costello or paid him a cent.
When questioning Mr. Cohen last week, the prosecution spotlighted his communications with Mr. Costello. Their emails and calls, Mr. Cohen said, left him with the impression that Mr. Costello was doing Mr. Trump’s bidding, delivering implicit instructions to, in the former fixer’s words, “stay in the fold, don’t flip, don’t speak.”
But Mr. Costello disputes that characterization, and Mr. Cohen’s testimony opened the door for the defense to call him to tell the other side of the story.
Mr. Costello, a former federal prosecutor turned defense lawyer, is expected to say that he encouraged Mr. Cohen to cooperate with prosecutors if he had anything truthful to offer. But, Mr. Costello has said, Mr. Cohen claimed he had no incriminating evidence to provide.
Mr. Costello will also likely testify that Mr. Cohen manufactured stories about Mr. Trump to save himself from prison.
Mr. Cohen, who pleaded guilty to federal charges in 2018, served more than a year behind bars. His crimes included orchestrating a hush-money payment to a porn star in the final days of the 2016 campaign, a deal at the center of the current state case against the former president. Mr. Cohen paid off the woman, Stormy Daniels, to silence her story of a sexual encounter with Mr. Trump.
The Cohen and Costello relationship began in the chaotic spring of 2018, after the F.B.I. searched Mr. Cohen’s home and office as part of the sprawling investigation.
They met through Jeffrey Citron, who was Mr. Costello’s law partner and Mr. Cohen’s acquaintance. Mr. Citron asked Mr. Cohen over email whether he wanted to connect with Mr. Costello and obtain “his insight into your situation, it would be my pleasure to arrange.” Mr. Cohen jumped at the offer: “I do. Can you connect me to him?”
When Mr. Trump hired Mr. Giuliani, Mr. Costello alerted Mr. Cohen to the potential upside: “I told you my relationship with Rudy which could be very very useful for you,” he wrote in an email. Mr. Cohen replied: “Great news.”
The records show that Mr. Costello pursued the relationship with Mr. Cohen.
As Mr. Costello lost patience, he wrote to Mr. Cohen complaining that he had “failed to communicate with me despite several text messages and emails.”
Mr. Costello also appeared to make a veiled reference to fear in Mr. Trump’s orbit that Mr. Cohen was poised to cooperate with prosecutors, writing: “We have continued our dialogue and need to bring you up to date concerning serious concerns on the part of our friends.”
Mr. Cohen was noncommittal. “When the right time comes, and now is not the right time, we will advance our conversations regarding this issue.”
But Mr. Cohen kept the channel open. For example, he sent Mr. Costello an article with the headline “Trump’s campaign to discredit Michael Cohen is already underway,” and added: “They are again on a bad path.”
And when Mr. Cohen’s relationship with Mr. Trump soured in summer 2018, he asked Mr. Costello to use Mr. Giuliani as a conduit for his assurances that he would remain loyal to Mr. Trump, according to Mr. Costello.
After Mr. Costello relayed the message, he told Mr. Cohen in an email that he had “conveyed all of your expressed concerns” to Mr. Giuliani “for transmission to his client” — the president. Mr. Cohen replied, thanking him.
Mr. Costello has said he was merely relaying messages from Mr. Trump’s legal team, including Mr. Giuliani, a former New York mayor. (Mr. Costello later represented Mr. Giuliani in criminal and congressional investigations.)
In one email to Mr. Cohen, Mr. Costello wrote, “Sleep well tonight, you have friends in high places.”
But in the end, no one remained friends, and much of the tension stemmed from money.
Mr. Costello and Mr. Cohen had a falling out, partly over unpaid legal bills. Last year, Mr. Costello testified before the grand jury that ultimately indicted Mr. Trump, seeking to undercut Mr. Cohen’s credibility.
Mr. Costello also sued Mr. Giuliani over delinquent bills.
And when Mr. Trump stopped paying Mr. Cohen’s legal fees, their relationship imploded as well. Mr. Cohen vowed to turn on Mr. Trump, a promise he is now fulfilling on the stand.
