This feels intuitively obvious, but apparently is demonstrably true: people vote (and donate to politicians and political causes) less out of love for their own party than from fear of the other. Further, as party affiliation further polarizes and the two "sides" become less like one another, that "fear" has at least some policy-based rationale, which only accelerates the movement, which has been going on consistently since 1980.
Further, this is a self-propelling engine — politicians are well aware of this, and play to that fear. Politicians and their affiliated media punditry can say outrageous things about Those Other People and get away with it because it's a play to emotions, not reason.
In 1952, Adlai Stevenson told the Democratic National Convention, "What counts now is not just what we are against, but what we are for." Unfortunately, we seem to have abandoned that message, and today, even someone "for" something often frames it as being "for something cool that those other people are against."
2 political scientists have found the secret to partisanship, and it’s deeply depressing
A chilling new theory of American politics.
It goes well beyond political party differences. You can use fear of other things (Iran, immigrants…) to keep people in the fold.
Of course, conservatives would argue that on the liberal side fear of things like global warming is being used to keep people democrat.
+Kee Hinckley The stats seem to skew a bit toward GOP use (and buy-in) to the tactic, but the Dems certainly aren't clean-handed in this.