I think there is a key distinction to be made here between moral and political opposition to slavery. By the 1850s/1860s, there were a lot of Americans (particularly Northerners) who opposed slavery not because they thought it was immoral, but because they believed the institution warped the federal government and their political system.

In the view of many Northerners, slaveholders in the South had secured excessive political power through the Three-Fifths Compromise (granting them additional seats in the House of Representatives and additional votes in the Electoral College). Additionally, because each state sent two Senators to Congress regardless of population, Southerners were significantly over-represented in the Senate relative to their actual population. Historically, slaveholding Southerners (particularly Virginians) had also dominated the presidency and the Supreme Court despite being a minority of the overall country's population. Northerners called this the "Slave Power" - a term indicating the belief that slaveholders were unfairly over-represented in the federal government.

In turn, then, Northerners were unhappy that the federal government seemed to use its power in ways that benefitted slaveholders. Many looked back to 1820, when the federal government carefully began balancing the number of free and slave states admitted to the Union. Many northerners saw the Mexican-American War in the 1840s as an effort by the Slave Power to snatch up land that would be used for slavery; simultaneously, some were frustrated that the federal government seemed uninterested in aggressively pursuing claims to the Pacific Northwest (which would have been made free territory). Acts like the Fugitive Slave Law in 1850 and the Dred Scott Decision by the Supreme Court in 1857 seemed to violate the principle that Northern states could choose to ban slavery within their own territory. The Kansas-Nebraska Act also violated the Missouri Compromise (established in 1820) which had not permitted slavery north of the Missouri Compromise Line.

This isn't necessarily a comprehensive list, but it covers some of the main events. What matters is that Northerners were mostly opposed to slavery because it seemed to warp and control the doings of the federal government. Few Northerners supported the complete and immediate abolition of slavery throughout the country by the time the Civil War broke out. It took a couple of years before Northerners decided that doing so would be a powerful economic tool for winning the war.

This isn't to say that there weren't people who opposed slavery on moral grounds. Obviously there were, or else slavery never would have been abolished in the Northern states in the first place. And famously, Harriet Beecher Stowe's novel Uncle Tom's Cabin alerted many people to what living conditions were actually like for slaves. But I think in the decades preceding the Civil War, most antislavery attitudes were held for political, not moral, reasons.

If you'd like to read further, I believe the seminal work on this subject is Leonard L. Richards's The Slave Power: The Free North and Southern Domination, 1780-1860. Daniel Walker Howe's What Hath God Wrought: The Transformation of America, 1815-1848 has some detail on this subject as well, though that is a hefty tome that covers a lot of other topics that are less relevant.