The historian Arthur Schlesinger Jr. long claimed that American politics moved in cycles. Every few decades, he suggested, the nation witnessed a swing of the pendulum, with power and ideological influence shifting from liberals to conservatives, or from the public interest to the private, and back again. When Barack Obama took office in 2009, many pundits saw the pendulum swinging once again, ushering in a new era of Democratic dominance — the Age of Reagan giving way to the vaunted Age of Obama.
The last two years have upended this story; presidential politics seems more and more unpredictable. When it comes to the writing of presidential history, however, the trends are still easy to call.