Senate Majority Leader Mitch McConnell made a terrible mistake Tuesday night when he released word that he was in favor of impeaching President Donald Trump. This mistake goes well beyond a simple political miscalculation. It is the physical manifestation of Washington Republican hubris, wrapped in a fog of frightened groupthink.Announced the eve of the House impeachment vote in the pages of The New York Times, McConnell’s plan reportedly rested on the belief that impeaching the outgoing president “will make it easier to purge Mr. Trump from the party.” Published just after the No. 3 House Republican, Conference Chairwoman Liz Cheney,...