The presidential contest has in effect been entered upon in such a way that the union of the Republican party has become a paramount necessity. The policy of the Democratic party signifies either separation or re-establishment with slavery. The Chicago platform is simply separation. General McClellan's letter of acceptance is re-establishment with slavery. The Republican candidate is, on the contrary, pledged to the re-establishment of the Union _without_ slavery, and, however hesitating his policy may be, the pressure of his party will, we may hope, force him to it. Between these issues I think that no man of the Liberal party can remain in doubt. I believe I am consistent with my antecedents and my principles in withdrawing--- not to aid in the triumph of Mr. Lincoln, but to do my part toward preventing the election of the Democratic candidate. In respect to Mr. Lincoln, I continue to hold exactly the sentiments contained in my letter of acceptance. I consider that his administration has been politically, militarily and financially a failure, and that its necessary continuance is a cause of regret to the country. On the following day this correspondence took place: EXECUTIVE MANSION, WASHINGTON, Sept. 23, 1864. _Hon. Montgomery Blair._ MY DEAR SIR: You have generously said to me more than once that, whenever your resignation could be a relief to me, it was at my disposal. That time has come. You very well know that this proceeds from no dissatisfaction of mine with you personally or officially. Your uniform kindness has been unsurpassed by that of any friend, and while it is true that the war does not seem greatly to add to the difficulties of your department, as to those of some others, it is not too much to say, which I most truly can, that in the three years and a half during which you have administered the general postoffice I remember no single complaint against you in connection therewith. Yours as ever, ABRAHAM LINCOLN. POSTOFFICE DEPARTMENT, Sept. 23, 1864. MY DEAR SIR: I have received your note of this date referring to my offer to resign whenever you would deem it advisable for the public interest that I should do so, and stating that in your judgment that time has come. I now, therefore, formally tender my resignation of the office of Postmaster-General. I cannot take leave of you without renewing the expression of my gratitude for the uniform kindness which has marked your course toward me. Yours truly, M. BLAIR. _To the President._