THE 2014 midterms have featured many variables and one constant. Whether they’re running as incumbents or challengers, campaigning in blue or red or purple states, Democratic candidates have all been dragging an anchor: a president from their party. So when and how was it lost? When President Bush’s second-term job approval numbers tanked, despite decent-at-the-time economic numbers, the explanation was easy: It was Iraq, Iraq, Iraq. But nothing quite so pat presents itself in Obama’s case, so here are four partial theories instead. He gets blamed for Republican intransigence. This is the explanation that many Obama partisans favor, because it...