That may have been my forgetfulness on the Underground Railroad. You are right, it did end in Canada and actually a number of blacks settled in where my ancestors in Grey County, ON were in about the 1850s. My idea was that, if it is correct, many Northerners did not like blacks escaping the plantations to come up in to their areas and there was the school of thought that had abolishing slavery would put a stop to the escapes up north.
Virtually every Northerner understood that slavery was built into the US Constitution, and without that agreement on slavery, the United States could not have united, ever.
So the vast majority of Northerners, while opposed to slavery in their own states were willing to tolerate it in the South.
The issue then boiled down to Western Territories, like Kansas.
There a serious competition resulted between slavers and free-soil men -- slavers supported by Federal government but outnumbered by free-soil immigrants.
In the 1860 election, Republicans supported free-soil while Democrats wanted more accommodation to slavers.
Abraham Lincoln was the first ever anti-slavery man elected President and that was too, too much for Southern Fire Eaters who immediately began organizing for secession.