Suppose a Democratic strategist wrote an open letter to colleagues and clients analyzing this week’s election returns. “Republicans have very little to cheer about this election,” he said with some satisfaction. “But off-year elections are rarely harbingers of future performance. Democrats actually did quite well in 2003, to little effect in 2004.” Suppose further that the Democratic strategist signed his name to the letter and distributed it widely.And then suppose one more thing: that the Democratic strategist had not actually written those words but had lifted them wholesale from a liberal blog. (In fact, those words do come from such...