BARACK OBAMA has arrived at a historical moment enjoyed by only one other Democratic president since Franklin D. Roosevelt: he gets to deliver a second Inaugural Address.

The second inaugurals we remember bear witness to political realignment. The words of Roosevelt and Ronald Reagan, in particular, testify to the closing of one era and the opening of another. In 1936 and 1984, Roosevelt and Reagan each won big. Their triumphs consolidated political transformations that had been building for some time and allowed their respective parties to reset the nation’s political center of gravity.

Without the benefit of historical distance, how do we judge whether we are in the midst of such a realignment? Are the country’s deepest political instincts undergoing fundamental change? Coming up with an answer is not easy....