The same might be said of Mr. Obama’s autobiography, which is less a straightforward chronicle than a carefully framed coming-of-age narrative. He describes himself as a young man adrift, although few friends recall thinking him so lost. And he just might have overstated his youthful experimentation with marijuana.

(Last November, an Iowa voter asked if he, unlike Bill Clinton, had inhaled. Mr. Obama looked puzzled. “I never understood that line,” he said. “The point was to inhale.”)

He carries a reputation as a Natural, and insists on calm. He did not interview each prospective campaign aide, but he laid down a rule: No drama kings or queens welcome. He confides in only a handful of advisers, particularly David Axelrod, the campaign guru with the appreciation for Chicago-style politics, and rarely displays public agitation about the measuring stick of his profession, electoral wins and losses. Told in February that he had won the caucuses in Maine, an overwhelmingly white state that he had expected to lose, he nodded, mumbled “That’s great,” and turned back to a phone call.

He jokes with his Secret Service agents and carries his own bags off planes and buses. (In this fishbowl world, a candidate knows he is being studied; carrying your own bags can be good manners, good politics, or both.) He jogs to the stage with the cocky ease of a jock.

He favors moderate tastes, preferring organic tea to a tumbler of gin, salmon to steak, a fruit plate to fries. He jokes about tossing back a beer, but his tippling amounts to a swig or two, most often to try to prove to television cameras that he is a “regular guy.”

But his greenness as a candidate also shows. His debate performances tend toward the erratic, authoritative one moment, defensive and diffident the next. He waxes incandescent at rallies, but in the 18-hour days leading up to primaries, he can sound aloof and querulous before smaller audiences. Condescension can creep in. He suggested, for example, that his youthful travels to Asia and Europe had left him more knowledgeable than Mrs. Clinton or Mr. McCain about foreign affairs.

“When I speak about having lived in Indonesia, having family that is impoverished in Africa, knowing the leaders is not important,” he told a crowd. “What I know is the people.”