From a reader:

Sorry, Jonah, but Judge Posner is wrong on the line being apochryphal, and, I believe, on it reflecting Holmes’ view of TR, not FDR.. According to Geoffery C. Ward’s A First Class Temperament: The Emergence of Frankin Roosevelt (1989) xiii-xv; 803, three witnesses, including one of Holmes’ law clerk’s (Donald Hiss) and Holmes and Roosevelt disciple Felix Frankfurter, left accounts drawn on in support of the story.
As to who the subject of the observation, it’s a little hard to imagine anyone, even Holmes, having the hubris to call Theodore Roosevelt a “second class intellect.” Roosevelt’s prowess as a historian (Winning of the West, Naval War of 1812), natural scientist and fellow Harvard man would have made a dramatic contrast for OWH with the much less significant achievements of FDR prior to his election.
Hey, I may be a lib–but John McCain isn’t the only TR fan left.

Well, I’m going to have to hit the books. I do think the reader is right that it’s odd to say that TR had a first class temperament and a second class intellect. It seems to me you could just as plausibly reverse that.