Digby took a look at the Red State/Blue State breakdown of recent presidential elections
Then, she put up this map of the free states and the slave owning or slavery sympathizing states in 1860:
And suddenly, it all makes sense. The bulk of the blue states represent the Union. The bulk of the red states represent the confederacy. The states and territories usually won by Democrats were the ones against slavery and fought to preserve America. The states and territories most likely to be won by Republicans believed in slavery and fought to destroy the United States.
Keep that in mind as the next election season rolls around.