Depending on 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 the Oregon Territory, California, and the Territory of 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 the Department of the Interior, built up 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. He left office the most successful President since George Washington in the accomplishment of his goals.

On the other hand, Polk's critics charge that his underestimation of the Mexican War's potential for disunion over the issue of slavery and his lack of concern with matters relating to the modernization of the nation contributed greatly to the sectional crisis of 1849-1850 and, in the early 1850s, to the fragmentation of both major parties. Polk's critics accuse him of being too partisan to understand the dangerous depth of the emotions that might erupt over the expansion of slavery westward. He left the nation at the end of his term facing its greatest political and social crisis since the American Revolution. That crisis would progressively tear the nation apart in the twelve years between 1848 and 1860.