It is equally certain that a political party could not come into the field in a contest for the Presidency upon such a declaration, without drawing into the discussion the whole subject of slavery as a domestic institution, or a condition of society, both in States and Territories. The intention was to draw a well defined line between the relations of Congress to slavery in the States and the relations of Congress to slavery in the Territories. Yet in the excitements of a Presidential canvass, the Republican party of necessity gathered into its folds those who had been for years regardless of that distinction, and who assailed slavery in the regions which were under the legislative power of Congress for the purpose of assailing it everywhere. The campaign literature, the speeches, the discussions, which dwelt on “the irrepressible conflict” between slavery and freedom, and which proclaimed the issue to be whether the United States would sooner or later become a slaveholding nation or a free-labor nation—whether the Northern States were to remain free or to become slave States—set forth with great distinctness in the writings and the harangues, could have no other effect than to array the two sections of the Union in a bitter hostility, while in the South there were those who believed, or affected to believe, that the people of the North, if successful in electing a President upon this basis, would put forth all their efforts to destroy slavery everywhere, as an institution incompatible with the continued existence of freedom in the North. All this hazard might, however, have been encountered and parried if the Democratic party had been in a condition to nominate a suitable candidate upon a “platform” fit to be opposed to that of the Republicans, and capable of commending itself alike to Northern and Southern voters. But when this party assembled in convention at Charleston, on the 23d of April, 1860, it was in no condition to do any good to the Union or to itself. If Mr. Buchanan had been a younger man, and had been disposed to be a second time a candidate for the Presidency, he might have united his party upon a basis of action in regard to this dangerous matter of slavery in the Territories, that would have commanded the support of a sufficient number of States, Northern as well as Southern, to have elected him. But he was averse to any longer continuance in public life, and he was well aware how much Mr. Douglas had done which had tended to divide the Northern and the Southern wings of his party. On the 14th of April, 1860, he sent to Charleston the following letter, which put an end to the idea, so far as it may have been entertained, of his being regarded as a candidate for the nomination by the Democratic National Convention. [MR. BUCHANAN TO HON. ARNOLD PLUMER.] WASHINGTON CITY, April 14, 1860. MY DEAR SIR:—