Based upon historical results in Georgia runoff elections, many experts in Georgia politics believed that Johnson was the favorite to win the runoff election. University of Georgia political science professor Charles Bullock said that McKinney would probably lose because historically most incumbents forced into a runoff in Georgia do.
The passage suggests that the outcome of the runoff election is predicted to be favorable for Johnson due to his historical advantage in similar races, and the tendency of incumbents to lose such contests.