waving the bloody shirt

Waving the Bloody Shirt: Then and Now

According to Wikipedia, and what I learned in public school, the term "Waving the bloody shirt" is a pejorative phrase, used to deride opposing politicians who made emotional calls to avenge the blood of soldiers that died in the Civil War.
In other words, it's a metaphor for unhelpful, grievance politics.

Except that wasn't true.
The origin of the term was far different.