I think your argument about “northern working class men” is pretty weak. But, I have never heard that angle, and it is food for thought. Thanks.
David Wilmot: “Wilmot was instrumental in establishing the Republican Party in Pennsylvania. His opposition to slavery did not include the evolving abolitionist position of immediately ending the institution in the entire country. His views on race were instead related to defense of white free labor and, by today’s standards, could be classified as racist.”
Lincoln actually makes it very clear in his debates with Stephen Douglas that he is not an abolitionist. He doesn’t target the actual moral issue of enslaving blacks, but instead attacks the political and economic clout that gets wielded by slaveholders. The whole point of the “House Divided” speech was not to predict the Civil War, but to point out that the Union would either exist as 100% slave states or 100% free states. It would exist as 100% slave states because Dred Scott would be extended to free states, preventing free states from banning slavery, or because the free states would fight off Southern tyranny.
https://en.wikipedia.org/wiki/David_Wilmot
(source on the quote about Wilmot)
http://susannalee.org/sources/files/original/0ed9b737cc1bb3c9f7987826e7285cc0.pdf