A representative for Mr. Harris, reached Wednesday, called the accusations "desperate and absurd."
The allegation is laid out in a new court filing by Mr. Black's legal team, as he fights civil claims from a former Russian model, Guzel Ganieva, who's accused him of sexual assault. Mr. Black has said their yearslong affair was consensual and she tried to extort him. And in a countersuit last year, he said unidentified public relations specialists and an unnamed funder were supporting her litigation in a scheme to destroy his reputation.
In a filing late Tuesday, Mr. Black's attorneys accused Mr. Harris of embarking on a "coup and smear campaign" in a failed attempt to become Apollo's chief executive officer.
They allege Mr. Harris worked with public relations specialist Steven Rubenstein to seed stories in the media about Mr. Black's past business ties with sex offender Jeffrey Epstein. Apollo commissioned a review that showed Mr. Black paid Mr. Epstein $158 million for tax advice and financial services, but the report cleared him of impropriety. Mr. Black then arranged for another founder, Marc Rowan, to become CEO last year — leaving Mr. Harris on the outs.
Mr. Black has since stepped down as chairman of Apollo and receded from coveted roles including chairman of New York's Museum of Modern Art.
Now, Mr. Black's attorneys are demanding phone records as they look to prove that Ms. Ganieva also is working with Mr. Rubenstein — and by extension, has ties to Mr. Harris. An attorney for Mr. Rubenstein said the PR specialist has "absolutely no relationship" to Ms. Ganieva.
In their filing seeking documents, Mr. Black's lawyers insisted that phone records could "show communication between Ms. Ganieva's camp and Mr. Rubenstein's camp — between his accuser and the public relations team that works for his archrival — further demonstrating that Ms. Ganieva's claims are nothing but fabrications stitched together from whole cloth."
"It is not a stretch," Mr. Black's lawyers added, "to infer that someone with very deep pockets is supporting her and/or this lawsuit. Mr. Black is entitled to probe and prove up that inference."
The drama between Mr. Black, 70, and Ms. Ganieva has been escalating for months in explosive back-and-forth lawsuits — starting with revelations about Mr. Black's extramarital relationship.
Mr. Black, who's worth $12.3 billion, according to the Bloomberg Billionaires Index, has admitted to paying Ms. Ganieva millions of dollars to keep their affair secret, payments which ended when she accused Mr. Black of sexual assault.
Ms. Ganieva's lawyers have disputed Mr. Black's claims that her case is being supported by a funder as "pure speculation," recently asking a judge to throw out his lawsuit. She sued him last year for defamation, intentional infliction of emotional distress, gender-motivated violence and retaliation.
"The claims in Mr. Black's legal memorandum are baseless, untrue and totally unsupported," Jonathan Rosen, a spokesman for Mr. Harris, said in a statement.
"Mr. Harris does not know Ms. Ganieva, has never met or spoken with her or anyone representing her, has no financial or any other dealings with her or her representatives, and had no involvement of any kind in the filing of any claims by her. Simply put, Mr. Harris has nothing whatsoever to do with the deeply troubling situation Mr. Black finds himself in, and any statement or implication otherwise is unhinged at best."
Evan Farber, a lawyer for Mr. Rubenstein, also disputed Mr. Black's allegations.
"Mr. Rubenstein and his firm have had absolutely no relationship with Ms. Ganieva, past, present, formal or informal," Mr. Farber said in a statement. "These, and the rest of the claims filed by Mr. Black's attorneys, offer a concocted, ever-evolving conspiracy theory, packed with false information, and are not based in reality."