In the interview, Mr. Heilemann said he was still coming to terms with the accusations that have been lodged against his longtime collaborator. Mr. Halperin — who has denied allegations of groping and assault, but acknowledged pursuing relationships with ABC colleagues — worked at ABC until 2007, roughly a year before he teamed up with Mr. Heilemann for “Game Change” during the 2008 presidential campaign.
“The bare nature of the accusations are horrific and shocking and terrible,” Mr. Heilemann said. “These behaviors are not the behaviors that I witnessed, and they’re not consistent with the person that I thought I knew. That’s not an excuse. That’s just the truth.”
Mr. Halperin declined to comment for this article. The allegations against him, which have expanded in recent days as more women have come forward, place him in a group of media figures felled by claims of sexual misconduct in the wake of numerous allegations against the film mogul Harvey Weinstein.
They include the literary editor Leon Wieseltier, the New Republic publisher Hamilton Fish, the Vox editorial director Lockhart Steele and the head of Amazon Studios, Roy Price. Mr. Halperin’s alleged misconduct with younger subordinates has been described as predatory and a sign of endemic sexism in television news.
It remains unclear what will become of Mr. Halperin’s contributions to the scuttled third installment in the “Game Change” series. The pair had conducted roughly 300 interviews for the book in recent months, Mr. Heilemann said.
“We’ve done reporting that touches on important things both for history and for the current national dialogue,” he said.
For years, Mr. Halperin played the Washington insider to Mr. Heilemann’s freewheeling descendant of Hunter S. Thompson, whose likeness he uses for his Twitter profile.
The two achieved riches and prominence that most reporters only dream about. They moved through the crowds at rallies lit up by their own dedicated Showtime camera crew. Their employer, Bloomberg News, installed a huge promotional poster for their show featuring their images on the side of the Marriott in Des Moines, a popular hangout for campaign reporters.
“Game Change,” a No. 1 best-seller, sold nearly half a million copies; its HBO film adaptation won Emmys and Golden Globes. They wrote a sequel, “Double Down: Game Change 2012,” and appeared frequently on cable news; Bloomberg was said to be paying Mr. Halperin and Mr. Heilemann salaries in the range of $1 million.
Mr. Heilemann, who made his reputation at Wired, The New Yorker and New York magazine, dreamed up the idea for “Game Change” — a Hollywood-style account of a presidential election — while vacationing in Aspen, Colo. He pitched the idea to Mr. Halperin, an acquaintance from the campaign trail, during a ride to a John McCain rally in 2008.
“Mark was my friend,” Mr. Heilemann said. “I cared about him then, and I care about him now. It’s also the case that Mark wasn’t in the circle of my closest friends.”
Many of their interviews were conducted jointly. Asked if he could use that material for a prospective solo project, Mr. Heilemann replied: “T.B.D. It’s complicated.”
Showtime said that the network remained on good terms with Mr. Heilemann and that it was examining whether “The Circus” will go on without Mr. Halperin. NBC News, where Mr. Heilemann is a contributor, has severed ties with Mr. Halperin.
The men worked closely for nearly a decade, including a two-year stretch of near-daily contact for their Bloomberg TV show. What would Mr. Heilemann say to a skeptic who found it difficult to believe that he had been unaware of Mr. Halperin’s alleged misbehavior?
“I would never try to argue anybody out of their skepticism about anything,” Mr. Heilemann said. “All I can say is, I barely knew Mark in the period of time when he stands accused of doing these things. People are entitled to their opinions about what I should have known, or must have known, or whatever. But the timeline is what the timeline is.”
He added: “I hope that my reputation has not been damaged by this. I don’t feel like I’m crumpled in the corner in some way.”