Charles Krauthammer has his sayings wrong. An emotional political appeal, especially in the context of Southern Segregation politics, is not to “unfurl the bloody flag”, but rather to “wave the bloody shirt”. According to the website listed below:
In the years that followed the American Civil War, there was a debate over the fate of the defeated Confederate states. Abraham Lincoln actually favored a relatively easy treatment and readmission into the federal union while the so-called "radical republicans" in congress favored a more punitive treatment with troops occupying the southern states, what came to be called the reconstruction. Opposed to the "radical republicans" were the democrats who had supported compromise even before the war. This battle was fought both in the congressional halls and the elections that filled those halls and it became the habit of reducing each conflict to a single issue: The War. And “The War” was reduced to the slogan "waving the blood shirt". Many believed that the bloody shirt was just a symbol. In fact, while it was symbolic, there was also a real bloody shirt, a nightshirt, but still a shirt.
The (night) shirt in question belonged to a federal agent named A. P. Huggins who served as tax collector (always a popular job) and school superintendent in a town in Mississippi. One night he was dragged from his house and beaten by the local Ku Klux Klan and then ordered out of town. This incident was used to justify the occupation of the southern states by federal troops and ultimately for voting republican rather then democratic. Benjamin Butler (R-MA) waved the actual “bloody (night) shirt” from this episode in congress during a speech requesting that the president be given the right to send troops to southern states to enforce the federal authority. Subsequently other bloody shirts were used to raise the emotions of the voters rather than deal with the issues.