Depending upon whom one reads, Polk comes across as either a nearly great President or as a man who missed great opportunities. Clearly, his impact was significant. Polk accomplished nearly everything that he said he wanted to accomplish as President and everything he had promised in his party's platform: acquisition of Oregon, California, and New Mexico; the positive settlement of the Texas border dispute; lower tariff rates; the establishment of a new federal depository system; and the strengthening of the executive office. He masterfully kept open lines of communication with Congress, established an administrative press, and conducted himself as a representative of the whole people. Polk came into the presidency with a focused political agenda and a clear set of convictions about America's destiny. He left the office the most successful President since Washington in the accomplishment of his goals.

On the other hand, Polk's blind eye to the immorality of slavery and his nonconcern with issues essential to the modernization of the nation led directly, his critics charge, to the disintegration of both major parties and the sectional crisis of 1849 and 1850. His critics accuse Polk of being too old fashioned and too narrow-minded to understand the dangerous depth of the emotions that were about to erupt over the expansion of slavery westward. Instead of rushing headlong into war and then rejecting the Wilmot Proviso outright, Polk might have used his office to engage the issues raised instead of just ignoring them. His critics say that Polk was the last President popular enough to have had any hope of successfully addressing the slavery issue. Instead, he fumbled the ball in favor of territorial gains that could have been won by diplomacy or the mere passage of time. He left the nation at the end of his term facing its greatest political and social crisis since the American Revolution. That crisis would tear the nation apart in the twelve years between 1848 and 1860.