wave the bloody shirt: foment political strife by keeping controversies alive
This expression first appeared during the Reconstruction period following the Civil War. Republicans, the "party of Lincoln," frequently used incendiary rhetoric to remind voters that pro-Southern Democrats were responsible for the war. Keeping alive bitter feelings against the South was known as waving the bloody shirt. The expression arose out of an actual incident. A former Ohioan named A. P. Huggins, serving as a Mississippi school superintendent, was roused from his bed one night by the Ku Klux Klan, ordered to clear out of the state and, to underline the message, given a beating with a leather strap. Huggins reported this attack to the military authorities. Afterward, one of the army officers took Huggins's bloodstained nightshirt to Washington, D. C. and gave it to Rep. Benjamin F. Butler of Massachusetts. According to the story, Butler brandished the shirt while making a fiery speech to demand stronger actions against the Klan. There is some question about whether Butler actually displayed the shirt or only figuratively "waved" it. Either way, the phrase caught on.
Showing a victim's bloodstained clothing is an old tradition. Some word historians have suggested that the expression was inspired by Corsican blood feuds, when a murdered man was laid out on a bier surrounded by his weapons, and with his bloody shirt hanging above his head. The widow or another female mourner would eventually begin wildly waving the shirt around her head as an expression of her sorrow and to spur the other mourners into taking revenge. Another possibility is that the phrase alludes to the 1603 Scottish battle of Glenfruin. The widows of this battle rode on palfreys in front of James VI, displaying their husbands' bloody shirts in an effort to gain his support.
The expression's meaning later broadened to cover any sort of political rabble rousing. It's still popular with political commentators, especially when the topic is a current war or other violent event.