He was when he urged the States to ratify the Corwin Amendment during his first Inaugural address. There is even some evidence that he may very well have written it himself.
"I understand a proposed amendment to the Constitution—which amendment, however, I have not seen—has passed Congress, to the effect that the Federal Government shall never interfere with the domestic institutions of the States, including that of persons held to service. To avoid misconstruction of what I have said, I depart from my purpose not to speak of particular amendments so far as to say that, holding such a provision to now be implied constitutional law, I have no objection to its being made express and irrevocable."
Now that is a huge lie. There is absolutely no evidence that Lincoln wrote the Corwin Amendment.
“The Congressional session that began in December 1860, more than 200 resolutions with respect to slavery,[6] including 57 resolutions proposing constitutional amendments,[7] were introduced in Congress. Most represented compromises designed to avert military conflict. Senator Jefferson Davis, a Democrat from Mississippi, proposed one that explicitly protected property rights in slaves.[7] A group of House members proposed a national convention to accomplish secession as a “dignified, peaceful, and fair separation” that could settle questions like the equitable distribution of the federal government’s assets and rights to navigate the Mississippi River.[8]
On February 27, 1861, the House of Representatives considered the following text of a proposed constitutional amendment:[9]
No amendment of this Constitution, having for its object any interference within the States with the relations between their citizens and those described in second section of the first article of the Constitution as “all other persons”, shall originate with any State that does not recognize that relation within its own limits, or shall be valid without the assent of every one of the States composing the Union.
Corwin proposed his own text as a substitute and those who opposed him failed on a vote of 68 to 121. The House then declined to give the resolution the required two-thirds vote, with a tally of 120 to 61, and then of 123 to 71.[9][10] On February 28, 1861, however, the House approved Corwin’s version by a vote of 133 to 65.[11] The contentious debate in the House was relieved by abolitionist Republican Owen Lovejoy of Illinois, who questioned the amendment’s reach: “Does that include polygamy, the other twin relic of barbarism?” Missouri Democrat John S. Phelps answered: “Does the gentleman desire to know whether he shall be prohibited from committing that crime?”[7]
On March 2, 1861, the United States Senate adopted it, with no changes, on a vote of 24 to 12.[12] Since proposed constitutional amendments require a two-thirds majority, 132 votes were required in the House and 24 in the Senate. The Senators and Representatives from the seven slave states that had already declared their secession from the Union did not vote on the Corwin Amendment.[13] The resolution called for the amendment to be submitted to the state legislatures and to be adopted “when ratified by three-fourths of said Legislatures.”[14] Its supporters believed that the Corwin Amendment had a greater chance of success in the legislatures of the Southern states than would have been the case in state ratifying conventions, since state conventions were being conducted throughout the South at which votes to secede from the Union were successful—just as Congress was considering the Corwin Amendment.”