What does thoreau predict about slavery in the US?

Thoreau believes that only a campaign of civil disobedience will be enough to defeat the evils of slavery. Thoreau was a radical individualist with a profound distrust of the collective will, however it is expressed. This explains why he was so reluctant to join with abolitionist groups for many years. He was also skeptical of their strategy of trying to abolish slavery through democracy.
Democracy is a human invention and thus it is fallible. It was therefore both foolish and unrealistic to expect that democracy could be a suitable way of abolishing slavery. Even if it could be achieved this way, then what's to say that the people won't, in the future, vote to restore slavery? If you put your trust in democracy to deal with pressing moral issues, then this is what you're leaving yourself open to. Besides, democracy can all too often lead to a tyranny of the majority, with individual rights being trampled underfoot. Thoreau's definition of slavery is quite broad, as set out here in Walden, for instance:

I sometimes wonder that we can be so frivolous, I may almost say, as to attend to the gross but somewhat foreign form of servitude called Negro Slavery, there are so many keen and subtle masters that enslave both north and south. It is hard to have a southern overseer; it is worse to have a northern one; but worst of all when you are the slave-driver of yourself.

Government was an enslaver of the people, not a liberator. It seems, then, absurd to trust it with the abolition of slavery in anything but a formal, narrowly legalistic sense. Thoreau didn't live to see his predictions come terrifyingly true during the aftermath of Reconstruction, when the formal apparatus of slavery was dismantled only be replaced by slavery's substance in the shape of the Jim Crow laws.