A simple and deceptively tricky question: What does a president do?

If you had to put together the Help Wanted ad for the position of chief executive, what would you write? Something like: "CEO needed to supervise 3 million employees. Must be at least 35, native-born, willing to work at home. Spectacular public failures likely."

The presidency is the most famous job in America (with all due respect to Oprah), and probably the hardest. The country is currently trying to fill the position. We have three applicants still in the running. What we don't tend to do, despite obsessive attention to this contest, is talk much about what the job entails. We talk instead about hot-button issues, the latest gaffe, the new sound bite, the polls, the electoral map. Presidential campaigns glancingly deal with the institution of the presidency while focusing on the more urgent issue of winning.

The closest thing we've seen to a job description on the campaign trail has been the 3 a.m. phone call ad, a caricature of the president as the national guardian, and one that still doesn't quite tell you what a president does during working hours.

"There's endless months of debating about this job and almost no public discussion of what the job is," Robert Caro, the two-time Pulitzer Prize-winning biographer now working on his fourth volume about Lyndon Johnson, told me last week. "There's no other job like it. I'm sitting here watching Lyndon Johnson grapple simultaneously with riots in the streets, budget problems in Congress, are the Chinese going to come into Vietnam, what's going wrong with the model cities program, how are we going to get the funding for Head Start, what's Bobby Kennedy doing today, how are we going to blunt what he's saying?"

Such a job requires an enormously flexible mind. It can be overwhelming. Presidents can get lost in the weeds. Johnson wound up poring over bombing charts from Vietnam. Jimmy Carter was so detail-obsessed he reportedly personally approved requests to use the White House tennis court. Roger Porter, who teaches about the American presidency at Harvard, says that Carter also got enmeshed in the parking assignments at the Department of Interior, as well as the crucial issue of federal cotton-dust standards.

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Theodore Roosevelt, for one, believed in the idea of a strenuous presidency, assigning to himself the right to take any action not expressly prohibited by the Constitution. "I did not usurp power, but I did greatly broaden the use of executive power," he wrote in his autobiography. In a lovely turn of phrase, he argued that a president shouldn't "content himself with the negative merit of keeping his talents undamaged in a napkin."

What he didn't know at the time was that, compared to what the Executive Office of the President would someday become, his White House wasn't much more than a fruit stand.

Consider how TR became president. He served as vice president under William McKinley. When McKinley was shot by an assassin in Buffalo, N.Y., Roosevelt traveled there and was told that McKinley was -- Roosevelt's phrase -- "practically out of danger." So what did TR do as the president lay wounded? He went on vacation with his family.

He traveled to the Adirondacks, and embarked on "a long tramp through the forest." He climbed a mountain. Someone finally tracked him down in the wilderness and told him that the president's condition had worsened. Roosevelt made an intrepid all-night journey through the darkness on muddy roads to return to civilization. Now, you could buy TR's story that his vacation was meant to reassure the anxious public that the president wasn't in danger. But perhaps it just shows how low the stakes were, compared to today -- how the vice president wasn't really that critical a figure in national government in 1901 even when the president had bullet holes in him.

Keep in mind that early presidents had essentially no staff at all and would either recruit a family member to help out in the day-to-day operations or pay someone out of their own pocket. In 1857, the Congress finally appropriated money for the president to hire a secretary.