They're good at unifying because their beliefs don't come from a core set of values. They can just all go along with the most convenient message of the day. That, plus their method of silencing their opposition by calling them racist, bigoted, etc. gives the appearance of majority acceptance. Finally, they rarely compromise on their current message and the repblicans give in because they're too afraid of the negatives press from the left leaning media. It's a multi facetted approach that constantly push the country further and further left and establishes a new normal. Even centrists policies 10 years ago would now be considered far right. All of this of course is allowed by the folly of the people.
Speaking of folly, I like to reference a piece by Dietrich Bonhoeffer who was killed by the Nazis:
https://creativeconflictwisdom.wordpress.com/2014/03/02/of-folly-by-dietrich-bonhoeffer-1906-45/