An attorney had been practicing in criminal defense for eight years and had handled many difficult and complex trials. One week ago, the attorney was on his way to work when he witnessed a taxicab hit a bicyclist. The attorney stopped to see if he could render assistance and recognized the bicyclist as his stockbroker. Today, the stockbroker contacted the attorney, and the broker hired the attorney to represent him in a personal injury claim. Under the Model Rules, what rule prevents the attorney from representing the broker?