No, hardly an "afterthought".
It was there from the beginning -- indeed, before the beginning:
A convenient lie they told their foolish constituency. How do we know? These very same people voted for the Corwin amendment.
I see you are re-stacking the same wood; the pile does look bigger this time until you notice there are huge holes in it.
If the ole rail splitter, side splitter, nation splitter was here he would tell you to add some seasoned, straight-grained arguments. Something with some heft; some BTUs.
Your soggy, punky offering has been eat-up by termites. There is simply not enough oxygen to get it lit off.
Now the sawyer cries, “Rest!” I recommend you pause for breath.
The Republican party was born as the anti-slavery party -- anti-slavery is what killed off the old Whigs.
Some of the GOP leaders had the strangest way of expressing and demonstrating their heartfelt abolitionism.
GOP Senate candidate Lincoln in an address at Springfield, Illinois, on June 26, 1857 (CW 2:407; 408-09):
There is a natural disgust in the minds of nearly all white people, to the idea of an indiscriminate amalgamation of the white and black races....A separation of the races is the only perfect preventive of amalgamation, but as immediate separation is impossible the next best thing is to keep them apart where they are not already together... Such separation, if ever affected at all, must be effected by colonization... The enterprise is a difficult one, but 'where there is a will there is a way;' and what colonization needs now is a hearty will. Will springs from the two elements of moral sense and self-interest. Let us be brought to believe it is morally right, and at the same time, favorable to, or at least not against, our interest, to transfer the African to his native clime, and we shall find a way to do it, however great the task may be.
GOP Presidential candidate Lincoln an address at New York City, February 27, 1860 (CW 3:541):
In the language of Mr. Jefferson, uttered many years ago, "It is still in our power to direct the process of emancipation, and deportation, peaceably, and in such slow degrees, as that the evil will wear off insensibly; and their places be, pari passu, filled up by free white laborers. If, on the contrary, it is left to force itself on, human nature must shudder at the prospect held up.''Mr. Jefferson did not mean to say, nor do I, that the power of emancipation is in the Federal Government. He spoke of Virginia; and, as to the power of emancipation, I speak of the slaveholding States only. The Federal Government, however, as we insist, has the power of restraining the extension of the institution—the power to insure that a slave insurrection shall never occur on any American soil which is now free from slavery.
GOP President-elect Lincoln to Duff Green, December 28, 1860 (CW 4:162):
I declare that the maintainance inviolate of the rights of the States, and especially the right of each state to order and control its own domestic institutions according to its own judgment exclusively, is essential to that balance of powers on which the perfection, and endurance of our political fabric depends—and I denounce the lawless invasion, by armed force, of the soil of any State or Territory, no matter under what pretext, as the gravest of crimes.
GOP President Lincoln, Inaugural Address, March 4, 1861 (CW 4:262-63):
Apprehension seems to exist among the people of the Southern States, that by the accession of a Republican Administration, their property, and their peace, and personal security, are to be endangered. There has never been any reasonable cause for such apprehension. Indeed, the most ample evidence to the contrary has all the while existed, and been open to their inspection. It is found in nearly all the published speeches of him who now addresses you.I do but quote from one of those speeches when I declare that "I have no purpose, directly or indirectly, to interfere with the institution of slavery in the States where it exists. I believe I have no lawful right to do so, and I have no inclination to do so.'' Those who nominated and elected me did so with full knowledge that I had made this, and many similar declarations, and had never recanted them. And more than this, they placed in the platform, for my acceptance, and as a law to themselves, and to me, the clear and emphatic resolution which I now read:
"Resolved, That the maintenance inviolate of the rights of the States, and especially the right of each State to order and control its own domestic institutions according to its own judgment exclusively, is essential to that balance of power on which the perfection and endurance of our political fabric depend; and we denounce the lawless invasion by armed force of the soil of any State or Territory, no matter under what pretext, as among the gravest of crimes.''
Spoken like a true abolitionist.
And then there was General Grant. It is notorious that during the war, his wife Julia Dent Grant, visited him on multiple occasions, accompanied by one of her slaves, demonstrating abolitionism.