According to sources who can be expected to know, Dr. Maria Esposito is going to run in the mayoral election. But if Dr. Esposito runs, Jerome Krasman will certainly not run against her. Ttherefore Dr. Esposito will be the only candidate in the election.
The flawed reasoning in the argument above most closely parallels that in which an informant says that Rustimann was involved in the bank robbery, if Rustimann was involved, Jones was certainly not involved, and since these two are the only people who could have been involved, Rustimann is the only person the police need to arrest.