From Lincoln's speech at Cooper Union:
You charge that we stir up insurrections among your slaves. We deny it; and what is your proof? Harper's Ferry! John Brown!! John Brown was no Republican; and you have failed to implicate a single Republican in his Harper's Ferry enterprise. If any member of our party is guilty in that matter, you know it or you do not know it. If you do know it, you are inexcusable for not designating the man and proving the fact. If you do not know it, you are inexcusable for asserting it, and especially for persisting in the assertion after you have tried and failed to make the proof. You need to be told that persisting in a charge which one does not know to be true, is simply malicious slander.