That was a technique used by Reconstructionists to bring the south to tow after the civil war.

Congressmen LITERALLY waved the bloody shirt of a Union soldier to remind Congress of the horror that the south had wrought upon the Union. It was very effective.

Trump's defense thinks this is something bad....it isn't...the country needs to see the bloody results of Trump's insurrection and we NEED TO WAVE THE BLOODY SHIRT so that nobody in the country can deny the treasonous effects of Trump's actions.