Excerpts from Books and Wikipedia
"To James Knox Polk, the imperial destiny of the United States manifested itself plainly enough. But it would be the press, not a presidential oration, that fixed the term 'manifest destiny' for the American public. . . . The integration of Texas into the Union represented 'the fulfillment of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.' . . . Whoever invented it, the phrase 'manifest destiny' passed into the American language, an illustration of the power of the press to capture the popular imagination with a slogan in an age of communications revolution. 'Manifest destiny' served as both a label and a justification for policies that might otherwise have simply been called American expansionism or imperialism. . . . The Whig Party conceived of American development more in terms of qualitative economic improvement than the quantitative expansion of territory. As Henry Clay wrote to a fellow Kentuckian, 'It is much more important that we unite, harmonize, and improve what we have than attempt to acquire more." [Howe: Howe: What Hath God Wrought, p. 702-6]