I don't think he actually forgot. There is a tradition by some on the Right to leave slavery out as a core reason for the Civil War simply because they don't hold the view that Slavery was very important.
Nikki Giovanni gets to the point....
McDonnell knew exactly what he was doing. It wasn't an innocent mistake. It was classic Southern Strategy stuff. But when he started to notice a backlash... he tried to make amends.
Unfortunately, I don't see this as having much of an impact nationally. Stupid voters will still vote overwhelmingly Republican in the next two major election cycles, no matter what. If anything, this dust up makes McDonnell look like a more appealing candidate for White Conservatives if he has any national political ambitions. But stories like this do serve to show the mindset of those on the Right who are in leadership.
Like I mentioned on my Republican media page a year ago (see sidebar) there is this desire by some on the Right to re-establish some sort of new Confederacy in this Country... if not a physical one... definitely an ideological one.
Nice Huffpost commentary