This is a hard question to really answer objectively, and it's overloaded.

Slavery always had its opponents, and they weren't ever as prevalent as most would think.

It's really a matter of who in power is truly benefiting.

About 3/4 of the men that signed the Declaration of Independence owned slaves, and two of the first 10 presidents that never owned slaves were John Adams and his son, John Quincy Adams. That should really show you the notion that all WHITE men were created equal. Also, common white citizens weren't allowed to vote until Andrew Jackson was in office, a side note to add more perspective in this world. Northern politicians were trying to get an amendment passed before the secession to have slavery be a right of the American people.

The difference of the Northern economies and the development of industry is probably what really causes this contrast. Farming needs a large workforce of manual labor, and that is basically the only industry of the South.

The North had the majority of the population and had almost all of the railroads, steel mills(iron reserves and resources like coal), exportation, and factories. These industries required a skilled labor force. I'm not saying slavery didn't exist in those sectors, but it doesn't mirror farming in the same fashion.

People in the North during the war might have supported abolishment of slavery, but a staggering majority of them viewed people of color as inferior beings, as well as the South. The average slave ownership for white families before the Civil War was around 35%, and while it was blatantly the only real reason for the war, that's not why the succession escalated into a real conflict(if that makes sense). Six states that remained in the Union were slave states.

It's a really odd subject that somehow gives a polarized view of the North and South. The idea that there was an alarmingly high number of people wanting slavery abolished, but then you look at conditions of black people after slavery..those dots don't connect to help formulate that being sound logic...and I probably wouldn't have even submitted this if my phone didn't accidentally sent it, but I felt like it should at least have some answer, even though I'm not the best source for this. Edit-finished last paragraph and added a 'the' and such cuz accidentally hit send.

Well there's really no way to know, as we didn't have opinion polling back then. The best we can do is look at how intellectuals, journalists, and politicians talked about the issue.

But having said that...

I'd say southern politicians understood that slavery as a political institution was on its last legs as early as the 1830s despite its steady growth in the United States in earlier decades and the South's dependence on it economically.

They watched slave revolts happen in the Caribbean (Haiti in particular was frightening). And Mexican independence from Spain came with the freedom of slaves, which then lead Texas (predominantly white men who had brought slaves to the region) to rebel for independence in order to keep slavery intact...in fact, some politicians talked about using Texas as a "valve" to remove slaves from the United States into Mexico.

I realize that's not a popularity metric per say, but in terms of what politicians--and their voters--"liked" about slavery (not sure that's the right word...?) their opinions probably had more to do with its effects rather than the thing itself. Slavery gave politicians from the South more power than politicians in the North (via the 3/5ths compromise) and it made plantation owners in the South very rich (Slaves are valuable property and did most of the labor, etc).

Importantly, they most likely knew that they had lost the philosophical battle on the issue, but that mattered very little. Today, slavery is largely seen as a philosophical problem, not a political one, and that's a mistake. It wasn't an easy yes/no, approve/disapprove kind of thing. It was more of a pro/con, trade offs kind of thing. We find that repugnant today, which is a good thing, but we can't let our feelings distort how they saw it...And with this issue, you can see how much the effects of slavery mattered more than the institution itself did in looking at the post Reconstruction era.

After the Civil War, the North occupied the South (Reconstruction) to ensure former slaves didn't fall back into slavery. But when Reconstruction ended, Jim Crow took over the South, forcing former black slaves into segregated communities, sharecropping skyrocketed, blacks were prevented from voting (but were now counted 5/5), etc, etc.

Lawmakers in the South basically wanted to recreate the effects of slavery without actually having slavery. And that's an important distinction.

My guess is that talking about owning slaves became taboo in the South during Reconstruction and it was not even a thought to reinstitute it...but wanting to recreate "the glory days" of the Antebellum South--when whites dominated the society and didn't have to answer to blacks--certainly persisted into the 20th century.