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 2.6 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 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.

The early days

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, Congress finally appropriated money for the president to hire a secretary.

‘President needs help’

Everything changed with the New Deal. Franklin D. Roosevelt, supervising the vastly expanded bureaucracy, found himself swamped.

“The president’s task has become impossible for me or any other man,” said Roosevelt in January 1937. “A man in this position will not be able to survive White House service unless it is simplified. I need executive assistants with a ‘passion for anonymity’ to be my legs.”

“The president needs help” was the instantly famous phrase of the subsequent report produced by the Brownlow Committee, headed by Louis Brownlow, an expert in public administration. The committee suggested that perhaps a half-dozen senior advisers would make the White House a more effective operation. In 1939, Congress approved the creation of the Executive Office of the President (EOP). Today the EOP has about 3,000 staffers. That doesn’t include the 15 departments run by cabinet secretaries or any of the other agencies (CIA, NASA, etc.) that are part of the executive branch.

Richard Neustadt, author of the seminal text “Presidential Power and the Modern Presidents,” wrote: “No president can spread himself across the whole of the post-Rooseveltian government.”

So the president has to have help. And very well may have to hang on for dear life.

Making decisions

Experts on the presidency repeatedly returned to a central premise: A president needs to be good at making decisions, lots of them, on complicated matters. This may seem screamingly obvious — but consider how little most of us know about the decision-making skills of the three people still running for president. We know more about the way they dress than the way they decide.

The easy decisions get made before they reach the president’s desk. A president has to be good at sorting through different viewpoints, encouraging debate. Jamie Gorelick, who was deputy attorney general in the Clinton administration, said that a strong leader “has to beg for the bad news. Because they will not give it to you naturally.”

Bill Clinton was not known to be fond of making decisions. Leon Panetta, who served as Clinton’s chief of staff, praised his boss’s intelligence, his ability to handle the barrage of data. Panetta added: “Sometimes his biggest problem was when he made a decision, he would continue to ponder it. You know: ‘Did I make the right decision? Was it the right thing to do?’ I’d tell him, ‘You’ve made the decision, you have to move on, you have other decisions to get to.’ “

Worst of all is when a president makes a bad decision — and sticks with it.

David Frum, a former speechwriter for George W. Bush, argues that there’s a tendency for a president to overreact to the failures of his predecessor. Bush was determined to be good at making decisions, Frum said.

“Some people might say he made them too quickly,” Frum said, “and he stuck by them too long.”

A packed schedule

The Reagan Library faxed me three days in Ronald Reagan’s “daily diary” from 1982 and 1983. I asked for typical days. What jumps out is how tightly structured they were, how jammed with meetings, almost all with senior staff by his side. On one day, Reagan went from the residence to the Oval Office at 9:03 a.m.; on another day at 9:02 a.m.; on the third at 9 a.m. He punched out at 5:41 p.m., 5:07 p.m. and 5:04 p.m., respectively.

Never known for long hours, so detached from details that he didn’t recognize one of his own Cabinet secretaries, Reagan somehow succeeded in advancing his agenda.

The current president wanted to be like Reagan. He is similarly a creature of habit. He’s an early starter, usually arriving at the Oval Office at 6:45 a.m., according to his press secretary, Dana Perino. His chief of staff, Joshua Bolten, will be nearby, but will give him a few minutes to sip his coffee in peace.

A recent Wednesday was a typical day. The record shows that Bush had breakfast at 7:15 a.m. with the king of Jordan. He had his usual 8 a.m. intelligence briefing. He held meetings with senior staff and the secretary of defense. At 10:40, he motorcaded to the Capitol, where, at 11:05, he participated in a ceremony honoring heart surgeon Michael DeBakey. Back at the White House, his schedule included a photo op with organ donors, Perino said. At 2:10 p.m., he had a meeting with some business leaders, and at 2:30, he met with Republicans from Congress. At 3:35, he briefly addressed the media about National Small Business Week.

Invariably, Bush has an exercise period in the mix, and almost invariably, he stays at home in the evening. “His docking station, as it were, is the residence,” says Robert Draper, author of “Dead Certain,” a book about the Bush administration. It’s lights out at 9 p.m., or not much later.

Bush rarely goes out on the town. He seldom appears before audiences that aren’t carefully screened. By his own account, he is immersed in the war he began.

Job takes its toll

The last century is littered with failed or mediocre presidencies. The job nearly crushed men who once strode the landscape like titans. They self-destructed in some cases, or had no business being in the job in the first place. Caro talks about the aging of the presidents: In photos, you see the years getting etched on their faces with alarming speed. Presidents age in dog years.

Lawrence Wilkerson, a Republican who served as chief of staff to former secretary of state Colin Powell, is no fan of the current president (“A good aphorism to describe this presidency is the Do Nothing Presidency Except Screw Up,” he told me). But asked if the job is even doable, he said, “Not by a single man, not unless he’s Superman.”

With all due respect to Hillary Clinton’s signature slogan, no one is ever completely “ready on Day One” for the job of the presidency.

There’s no job like it. Anyone who becomes president is making a daring leap — and asking the country to make the leap simultaneously. To run for president is an ostentatious act.

Here’s what is certain: Next January, we’ll have a new president. And the president will need help.