A lot of people who look at those Democrat-Republican red and blue maps don't get it.
In many ways those maps are similar to the maps of the 1850s before the great American Civil War. The country was divided. One of the reasons it was a north-south division is because much of the west was still territory. Statehood lay far in the future for states like Arizona and New Mexico.
A lot of people get sidetracked by what many historians have painted as the major issue behind that 1850s division, slavery. As horrible and as ugly as any form of slavery is, the real underlying cause, as it almost always is for war, was economic: One section of the nation trying to force its policies on another.