The sentiment of opposition to the Slave Power, to the extension of Slavery, and to its longer continuance, wherever under the Constitution the National Government is responsible for it, though recognized by individuals, and adopted by a small and faithful party, is now for the first time the leading principle of a broad, resolute, and national organization. It is, indeed, as Mr. Webster lately said, no new idea; it is old as the Declaration of Independence. But it is an idea now for the first time proclaimed by a great political party: for, if the old parties had been true to it, there would have been no occasion for our organization. It is said, our idea is sectional. How is this? Because the slaveholders live at the South? As well might we say that the tariff is sectional, because the manufacturers live at the North. It is said that we have but one idea. This I deny. But admitting that it is so, are we not, with our one idea, better than a party with no ideas at all? And what is our one idea? It is the idea which combined our fathers on the heights of Bunker Hill,--which carried Washington through a seven years' war,--which inspired Lafayette,--which with coals of fire touched the lips of Adams, Otis, and Patrick Henry. Ours is an idea at least noble and elevating; it is an idea which draws in its train virtue, goodness, and all the charities of life, all that makes earth a home of improvement and happiness. "Her track, where'er the goddess roves, Glory pursue, and generous Shame, The unconquerable Mind, and Freedom's holy flame." We found now a new party. Its corner-stone is Freedom. Its broad, all-sustaining arches are Truth, Justice, and Humanity. Like the ancient Roman Capitol, at once Temple and Citadel, it shall be the fit shrine for the genius of American institutions. PARTIES, AND IMPORTANCE OF A FREE-SOIL ORGANIZATION. LETTER ADDRESSED TO A COMMITTEE OF THE FREE-SOIL PARTY IN BOSTON, OCTOBER 26, 1848. In the political campaign which followed the nominations at Buffalo Mr. Sumner took an active part, addressing large audiences at all the principal places in Massachusetts, beginning at Plymouth. On these occasions he discussed at length the failure of the two old parties, and the political character of their candidates, especially in contrast with those of the Free-Soil party, vindicating the necessity of political action against the Slave Power and the extension of Slavery. Contemporary newspapers show the impression produced, and, in the absence of any authentic report, are quoted here. Of his address at Springfield one of his hearers says in a newspaper:--