Well, obviously slavery is a major theme in this speech—it's basically the entire point of the speech. Slavery is, after all, why the "house" is "divided." The way Lincoln talks about it, though, is mostly quite calm and pragmatic, compared to speakers like Frederick Douglass.
Lincoln spends most of his speech explaining the current situation of slavery and the slavery debate in the U.S., and why the North is in danger of having to accept the institution in their midst. Despite the logical nature of his description, the future president is clearly giving slavery a big ol' thumbs down.
Lincoln stays away from emotional statements on slavery and morality, probably because he didn't know how that would fly with his audience.
It's interesting how both this speech and the outbreak of the Civil War are related to the question of states' rights to make decisions about slavery, coming from opposite sides of the conflict.
In the antebellum period, politics and the legal system became super intertwined over the issue of the expansion of slavery. The way slavery was regulated was the ultimate talking point—or rather, shouting point—of the 1850s, and finally became an issue for the Supreme Court.
In his "House Divided" speech, Lincoln muses on the role of the U.S. government and the legal system in the slavery debate, rather than the public. He looks at how the Constitution in particular has been used to protect the institution of slavery. (Which: boo.)
Maybe it's not a coincidence that the lawyer guy is so interested in the judicial system.
Lincoln and Taney seem to have very, very different opinions on what the Constitution says.
We all love a nice walk down memory lane, but Lincoln's focus on the past is less nostalgic than practical. Much of the "House Divided" speech recounts the past few years to show how the U.S. has arrived at the precarious situation it was in by June 1858. Lincoln's argument relies on convincing people of danger based on the progression of events as evidence.
So it's less of a walk down memory lane than using the past to prove his point. Less emotionally enjoyable, but even more highly effective.
Lincoln probably recounts very recent events because people have a tendency to tune out when too many bad things are happening, and he wants to make sure they're thinking about it.
Given that the slavery debate had changed in recent years, there wasn't much precedent to guide people in figuring out the future, so they kept worrying about the past.
It's pretty clear that Lincoln doesn't like the divided house. He's not standing up in front of the Republicans saying "guys, this is awesome, let's keep it up." A lot of his dissatisfaction with the current situation, unsurprisingly, directed towards Stephen Douglas, his imminent rival.
Some of the more subtle lines in "House Divided" could be not-so subtle depending on the delivery, but sadly, we weren't here in 1858 and can't be sure how dripping Lincoln's voice was in derision. It's still very clear, though, that the future president has some strong feelings about the state of things in 1858 America.
If only people had discovered the healing power of the song "Kumbaya" by 1858, we might not have a "House Divided" speech, because everyone would be friends again and Lincoln would be happy.
If they ever make a movie featuring this speech, there's a really great Rolling Stones song that Lincoln should walk on to. Take note, Hollywood.
A politician playing on his audience's fears? No, that can't be right, they would never do such a thing. Except all the time, always.
In this case, Lincoln shares the fear of his audience: that the northern states who have basically never wanted slavery will soon be forced to allow it by the Supreme Court. The divided house of the "House Divided" speech will have to unify one way or the other, and one of his big points is that it really could go either way.
In fact, he spends most of the speech convincing his listeners how likely it is that the court will choose the option they don't want, to motivate them to stay strong against not just pro-slavery forces, but the doctrine of popular sovereignty as well.
"The only thing we have to fear, is fear itself"…unless you're an American who doesn't like slavery in 1858. In that case, there are one or two other things.
Some of the pro-slavery folks were just as scared that they would lose their right to own slaves, but of course we're not nearly as sympathetic towards those guys.