Arthur M. Schlesinger Jr., The Age of Jackson
(1945), XXXIV, 1 (p. 452):
Polk's great defect was an inability to recognize the honesty of opposition. It was always selfish and factious, based on ambition or jealousy or disappointment over patronage. This dry assumption of infallibility gave his administration its peculiar strength -- its decision, its firmness of purpose, its steady selection of ends, and its precise achievement of them. But it was also responsible for its peculiar weakness. As a result of misjudging the grounds of opposition, Polk consistently underestimated its moral force.