Lincoln first informed his cabinet in July of 1862 that he had decided to issue the Emancipation Proclamation, which freed all slaves in the rebellious states but which left slavery in tact in the border states, as a punitive measure to hurt the Confederate war effort.
Lincoln's cabinet was made up of more tenured leaders of the Republican party than himself, a party founded on the platform to abolish slavery. So these self proclaimed abolitionists should have welcomed Lincoln's impending executive order. At least one would have thought.
None of Lincoln's cabinet supported the Presidential order and all argued strongly against it. Secretary of the Treasury Salmon P. Chase was against, afraid Lincoln's punitive and limited abolition in the Southern rebellious states might lead to universal emancipation, saying that “it goes beyond anything that I have recommended.”
Secretary of State William H. Seward gave an impassioned speech warning that “foreign nations will intervene to prevent the abolition of slavery for sake of cotton.” Postmaster-General (yes that was a cabinet position back then) Montgomery Blair, was against it, saying that it would cost the administration the fall elections. Still others were against it because they believed Lincoln did not possess the authority.
In the end Lincoln's cabinet, particularly Secretary of State William Seward, persuaded him to postpone based upon a more pragmatic argument. “Postpone its issue, until you can give it to the country supported by military success, instead of issuing it, as would be the case now, upon the greatest disasters of the war! (McClellan’s failed Peninsula Campaign)”. Lincoln would wait for a battlefield victory and use the Emancipation Proclamation as a follow up blow – the first on the field of battle, the second on the fields of the plantations. ( from Team of Rivals, Doris Kearns Goodwin).
September 1862, after the Union victory at Antietam, Lincoln announced his intent to the nation.... Lincoln then formalized and signed the presidential order the Emancipation Proclamation, Jan 1 1863.
The 13th Amendment to the Constitution was necessary firstly because The Emancipation Proclamation (executive order), was likely to be declared null and void if the rebellion ended and a Supreme Court in more settled times considered the ramifications of Lincoln's unilateral and unsanctioned by congress act, along with the rest of his usurped wartime powers. It was possible Emancipation Proclamation could be repealed by a stoke of a single pen by a future President elected potentially with Southern votes in a post war Union. Finally, because the Emancipation Proclamation that Lincoln signed did not end slavery in the United States, but only in the rebellious Southern states. Slavery still existed at the end of the war in the border states which had not rebelled including: Delaware, Kentucky, Maryland, Missouri and West Virginia. For those states the 13th amendment is what ended slavery, not the Emancipation Proclamation.