Stephen Meyers (Ryan Gosling) is the Junior Campaign Manager for Mike Morris (George Clooney), Governor of Pennsylvania and a Democratic presidential candidate, competing against Arkansas Senator Ted Pullman (Michael Mantell). The candidates are campaigning in Ohio. A win for Morris would all but guarantee him the nomination; a win for Pullman would give him vital momentum. Both campaigns are also attempting to enlist Senator Thompson’s (Jeffrey Wright) endorsement.
After a debate, Meyers is asked for a secret meeting by Pullman’s Campaign Manager Tom Duffy (Paul Giamatti). Meyers calls his boss, Senior Campaign Manager Paul Zara (Philip Seymour Hoffman), who doesn’t pick up. Meyers leaves a message that something important has come up. Meyers meets with Duffy who offers him a position in Senator Pullman’s campaign. Meyers refuses, and asserts that he believes in Morris. Duffy tells Meyers that his optimism won’t last, and that Morris will eventually be cynical and corrupt like the other candidates. Zara calls Meyers back and asks what was important but Meyers says it was nothing to worry about.
Meyers starts a sexual relationship with Molly Stearns (Evan Rachel Wood), an intern for Morris’ campaign and daughter of Jack Stearns (Gregory Itzin), the chairman of the Democratic National Committee. Late one night, Meyers discovers that Morris is trying to call Molly. She and Morris had a brief sexual liaison, at a campaign stop in Iowa several weeks previously, and Molly is pregnant with Morris’ baby. Meyers helps her with money for an abortion and drives her to the clinic.
Meyers admits to Zara that he met with Duffy, who told Meyers that Pullman will offer Senator Thompson the position of Secretary of State, guaranteeing his victory by bringing hundreds of delegates with him. Ida (Marisa Tomei), a reporter for the New York Times, reveals that an anonymous source leaked his encounter with Duffy to her and that she will publish unless Meyers gives her all of the information about his meeting with Thompson. Meyers comes to Zara for help. Zara reveals that he leaked the meeting to Ida and fires Meyers from the campaign for showing a “lack of loyalty” in meeting with Duffy.
Meyers offers his services to Duffy but Duffy doesn’t want to hire Meyers. In a Machiavellian scheme, Duffy admits he met with Meyers in order to induce Meyers to tell Zara about the meeting. Duffy correctly predicted that this would lead Zara (in his paranoia) to remove Meyers from Morris’ campaign, thus weakening Morris. Meanwhile, Molly learns that Meyers has been fired and, fearing that her secret will now be exposed once he leaves, commits suicide by overdosing on pills. Meyers feels guilty about this, as he did intend to expose Morris’ affair with Molly in exchange for a job on Senator Pullman’s campaign.
Meyers later confronts Morris and tells him that he will expose the affair with Molly if Morris does not replace Zara with himself, as well as offer Senator Thompson a place on the ticket, guaranteeing Thompson’s support. Morris relents when Meyers claims he has a suicide note which he says he stole from Molly’s room. Later at Molly’s funeral, Zara compliments Meyers on using his own secrets to his advantage. Having accepted Thompson’s endorsement and his delegates, thereby giving him enough delegates to gain the nomination, Morris becomes the nominee despite losing the Ohio primary to Pullman.
Now Senior Campaign Manager, Meyers attends a press conference organized by Ida. He realizes that Duffy was right, and he has betrayed everything he believed in for success and revenge. The movie ends as he takes his seat for a television interview.