What prediction did Abraham Lincoln make in his debates with Stephen Douglas?

In 1858, Republican Abraham Lincoln challenged Stephen Douglas for the Senate seat in Illinois. Lincoln opposed the spread of slavery in the West, while Douglas was the leading supporter of popular sovereignty. During the campaign the two men held a series of seven debates focusing on the crisis over slavery. In the first debate, Lincoln (whom even Douglas called the best political speaker in the West) declared famously that "a house divided against itself cannot stand." As Lincoln put it, the U.S. could not remain "half slave and half free": one side or the other must prevail. Douglas accused Lincoln of advocating complete equality between black and white Americans, a charge that Lincoln denied. In the fall election, Douglas defeated Lincoln.