Before the nomination of Buchanan and Breckinridge another Presidential ticket had been placed in the field. The pro-slavery section of the American party and the ghastly remnant of the Whigs had presented Mr. Fillmore for the Presidency, and had associated with him Andrew Jackson Donelson of Tennessee as candidate for the Vice-Presidency. On the engrossing question of the day Mr. Buchanan and Mr. Fillmore did not represent antagonistic ideas, and between them there could be no contest to arouse enthusiasm or even to enlist interest in the North. The movement for Fillmore afforded a convenient shelter for that large class of men who had not yet made up their minds as to the real issue of slavery extension or slavery prohibition. The Republican party had meanwhile been organizing and consolidating. During the years 1854 and 1855 it had acquired control of the governments in a majority of the free States, and it promptly called a national convention to meet in Philadelphia in June, 1856. The Democracy saw at once that a new and dangerous opponent was in the field,--an opponent that stood upon principle and shunned expediency, that brought to its standard a great host of young men, and that won to its service a very large proportion of the talent, the courage, and the eloquence of the North. The convention met for a purpose and it spoke boldly. It accepted the issue as presented by the men of the South, and it offered no compromise. In its ranks were all shades of anti-slavery opinion,--the patient Abolitionist, the Free-Soiler of the Buffalo platform, the Democrats who had supported the Wilmot Proviso, the Whigs who had followed Seward. NOMINATION OF JOHN C. FREMONT.