The Showtime political documentary series “The Circus” is set to return in April, minus its most recognizable star: Mark Halperin, the high-flying journalist whose career was felled last year by allegations of sexual harassment and sexual assault.

Replacing Mr. Halperin will be Alex Wagner, a CBS News anchor and former MSNBC host. She will join two of the original hosts, the writer John Heilemann, Mr. Halperin’s longtime professional partner, and Mark McKinnon, a Stetson-sporting political consultant.

“The Circus” is the only project linked to Mr. Halperin to survive the numerous allegations against him, including claims that he made unwanted sexual advances toward female co-workers at ABC News. Mr. Halperin lost his gig as a commentator at MSNBC, and Penguin Press canceled the next installment of “Game Change,” the lucrative franchise of campaign books he wrote with Mr. Heilemann.

HBO, one of Showtime’s rivals, also canceled a planned “Game Change” movie about the 2016 presidential race.