The way you divide a country is you turn every single issue into a straight, binary issue and tell the people to choose good vs. evil. As I said the other day, in modern politics every decision a president makes is viewed simultaneously as a huge and important triumph and a catastrophic failure that forewarns the end days. Most issues have pros and cons with plenty of gray in between, but that gray costs you elections in an environment where the voters only want the simplest choices with no cause for thought.
And this, by the way, was Rove's single greatest contribution to America. He certainly didn't invent the strategy, but he mastered it to a degree that would have given any Roman serious wood.