I would say the romance is a major focus of the plot - the main couple is reunited because one is the only person with the murder victim as he dies, and their rekindled relationship keeps pace with their solving the murder. However, there is no doubt that it is a mystery first and foremost (however much the romance between the detectives is the reason for the story's charm) and it might be easier for you to keep this exchange to works that can be classified primarily as romances.