Mr. McCain, we’re told, is a straight-talking maverick. But on domestic policy, he offers neither straight talk nor originality; instead, he panders shamelessly to right-wing ideologues.

Mrs. Clinton, we’re assured by sources right and left, tortures puppies and eats babies. But her policy proposals continue to be surprisingly bold and progressive.

Finally, Mr. Obama is widely portrayed, not least by himself, as a transformational figure who will usher in a new era. But his actual policy proposals, though liberal, tend to be cautious and relatively orthodox.

Do these policy comparisons really tell us what each candidate would be like as president? Not necessarily — but they’re the best guide we have.