According to The Hollywood Reporter, Harrelson will play Steve Schmidt, John McCain's senior campaign strategist and advisor in the forthcoming TV project.
The film, based on John Heilemann and Mark Halperin's book of the same name, tells the story of the 2008 presidential race and centres on McCain's story in particular, as well as his eventual defeat to Barack Obama.
Ed Harris recently landed the role of McCain in the film.
Julianne Moore previously signed to play McCain's campaign running mate and former Alaskan governor Sarah Palin.
Jay Roach is helming the project from a script by Danny Strong. The pair also collaborated on the Emmy-winning HBO movie Recount, which centred on the 2000 US Presidential election.