Yesterday, speaking at Northwestern University’s much-esteemed Kellogg School of Management, President Obama committed a huge strategic blunder. It has been no secret that Democrats running for Senate seats have been distancing themselves from the unpopular president, often finding excuses not to greet him on the tarmac or appear next to him on the podium when he visits their states. For them, it is imperative that voters separate support for Kay Hagan or Al Franken (for instance) from support for Barack Obama. They want the midterm election to be a local affair, not a national referendum. But President Obama just undid...