What is the main conflict in The Widening Gyre by Robert B. Parker?

Boston-based private detective Spenser goes to work for a conservative senatorial candidate named Meade Alexander, allegedly to protect him from some vague threats he has received. However, Meade reveals that Spenser will be investigating a blackmail tape showing Ronni, Meade's wife, having an affair with an anonymous man. Meade's opponent in the political race, Robert Browne, is high on Spenser's suspect list because Browne is on the payroll of Boston mafioso Joe Broz, who does not want to see his politician get beaten by a conservative whose wife has been indiscreet. Spenser empathizes with Meade, whose only wish is to protect the woman he loves.