If you honor a flag raised explicitly to destroy this country then this is the movement for you.
Defenders of the Confederate flag and other efforts to honor the Old South always say they're not interested in slavery or racism but heritage. Let's leave aside how the racism and slavery are inextricably bound up in that heritage; we'll ignore them entirely. (Although Republicans who chafe under the burden of racism accusations might stop and consider, for a moment, how actions like McDonnell's look to African Americans.)
Even putting its best foot forward, the reason the Confederacy existed was to tear asunder the United States of America. You can't get around it.
In that sense, the Confederates who fired on Fort Sumter weren't all that different from the Japanese who bombed Pearl Harbor, the Germans who sunk the Lusitania or the hijackers who hit the Twin Towers and the Pentagon on 9/11. We don't raise memorials in their honor, we don't fly their flags and we don't make proclamations in their memory -- their actions were an assault on the United States and its citizens.
Honoring the Confederacy, then, is a signal of contempt for the United States of America. Period.
Not all - probably not even most - Republicans are lovers of the Confederacy. But Confederacy-loving sentiment mostly finds its home in today's Republican Party. There is some irony here, since the GOP likes to style itself as more-patriotic-than-thou. But in the words of a Republican president: "You're either with us or against us." How can you love this country and the people who tried to destroy it? It doesn't make any sense.