This conversation took place (assuming it is relayed accurately) in the dark days of August 1864, when Grant's assault on Lee's army was stalled with appalling slaughter, Sherman had not yet taken Atlanta, and a wave of war weariness was overcoming the electorate. Lincoln's honest parsing of personal ambition and love of country at the outset is itself remarkable, if characteristic, but his read on the military/political/economic forces at work is even more remarkable -- at least to this moderately informed reader. The source below is Hay's short version of the ten-volume biography he wrote with Nicolay. I don't know whether the ellipses appear in the full biography.
Mr. Lincoln realized to the full the tremendous issues of the campaign. Asked in August by a friend who noted his worn looks, if he could not go away for a fortnight's rest, he replied: "I cannot fly from my thoughts—my solicitude for this great country follows me wherever I go. I do not think it is personal vanity or ambition, though I am not free from these infirmities, but I cannot but feel that the weal or woe of this great nation will be decided in November. There is no program offered by any wing of the Democratic party, but that must result in the permanent destruction of the Union."