Nate Silver blogs at the NYT's 538.

With President Obama’s second term under way, we have begun to see more reflections on how he might come to be regarded historically.

As common sense might dictate — and as the statistics will also reveal — it is far too soon to conclude very much about this. Second-term presidents may be derided as lame ducks, but it is often in the second term when reputations are won or lost.

Still, we can say this much: Mr. Obama ran for and won a second term, something only about half of the men to serve as president have done (the tally is 20 or 21 out of 43, depending on how you count Grover Cleveland). We can also note, however, that Mr. Obama’s re-election margin was relatively narrow. Do these simple facts provide any insight at all into how he might be regarded 20, 50 or 100 years from now?

In fact, winning a second term is something of a prerequisite for presidential greatness, at least as historians have evaluated the question. It is also no guarantee of it, as the case of Richard M. Nixon might attest. But the eight presidents who are currently regarded most favorably by historians were all two-termers (or four-termers, in Franklin D. Roosevelt’s case)....