If that 3 a.m. call rings in the White House after Jan. 20, much has been made over which candidate would have the wisdom and experience to handle it.
But what about the stamina?
The health and fitness of presidential candidates has come under increasing scrutiny over the past three decades, with the release of medical records, interviews with the candidates' personal physicians and a microscopic look at any condition that has afflicted the person running for office.
This year's race adds another dimension: John McCain, if elected, would be the oldest first-term president at 72. Barack Obama is 47. While McCain's age or health has not been a major campaign issue, the candidate has pointed to his 96-year-old mother, Roberta, as proof of his good genes.
''All the candidates these days have to release their records,'' says Robert Dallek, author of several presidential biographies, including An Unfinished Life: John F. Kennedy, 1917-1963 (Little Brown, 2003). "There's a great deal of public anxiety about it.''
In May, each candidate released information about their medical histories. The McCain campaign made available 1,173 pages of his medical records from 2000-2008 to 20 reporters. In a conference room, they reviewed the documents over a three-hour period; they could not remove the papers or photocopy them.
The reporters then had a conference call with McCain's doctors, who said McCain was in ''excellent health.'' The doctors said the melanoma in McCain's left temple, a potentially fatal form of skin cancer removed during surgery in 2000, had not recurred. The records also indicated McCain takes a statin for cholesterol, aspirin to prevent blood clots and medication to prevent kidney stones.
A week later, Barack Obama issued a six-paragraph, one-page letter signed by his physician of 21 years, Dr. David Scheiner. Scheiner's letter indicated Obama was in "excellent health.''
The one issue noted in the letter was Obama's effort over the years to quit smoking, a habit he began more than 20 years ago. Scheiner, who did not speak to reporters, wrote Obama is ''using Nicorette gum with success.'' The letter did not specify how much Obama, who quit smoking in 2007, had smoked on a daily basis. (McCain smoked for 25 years, quitting in 1980.)
Releasing detailed medical records has become the norm for presidential and vice presidential candidates. It's a direct result of the presidential campaign of 1972, when Democratic vice presidential candidate Thomas Eagleton, a U.S. senator from Missouri, concealed, then revealed his treatments for depression. He resigned from Sen. George McGovern's ticket 18 days after accepting the vice presidential nomination.
''The watershed moment was over Tom Eagleton in the 1972 campaign,'' said David Greenberg, a presidential historian at Rutgers University and author of Nixon's Shadow: The History of an Image (W.W. Morrow, 2003).
Ronald Reagan, faced with concerns in 1980 that he would be the oldest person to be elected president at age 69, said in an interview with The New York Times in June 1980 that if he were elected to the presidency that November, he would have the White House physician periodically check his physical and mental state. He said he would resign if evidence demonstrated he was unfit. Reagan's mother, Nellie, had become senile a few years before she died at age 80.
Reagan's doctors at the time said he was in excellent health. Reagan had three major surgeries during his two terms in office, including having a bullet removed from his lung after he was shot on March 30, 1981. Besides that emergency procedure, he had a malignant polyp removed from his bowel in 1985 and a prostate resectioning in 1987. (He also had three cancerous skin lesions removed from his nose between 1985 and 1987.)
Reagan also occasionally dozed off in Cabinet meetings and had periods of forgetfulness. He left office in 1989, announced his Alzheimer's diagnosis in 1994 and died in 2004.
George H.W. Bush, who succeeded Reagan in 1989 at age 64, released medical records during his campaign and yearly during his presidency. His doctors said he had mild degenerative osteoarthritis in his hips, normal X-rays, EKG, urinalysis and blood work.
In 1991, while jogging at Camp David, Bush suffered shortness of breath and was taken to the hospital by helicopter. His doctors diagnosed atrial fibrillation, a rapid, irregular heartbeat. Doctors said the condition was treated with drugs.
Bill Clinton, 46 when he assumed the presidency in 1993, released medical records during his campaign and his presidency. Doctors said he was overweight and prescribed Zocor for high cholesterol. (Heart disease runs in his mother's family.) He jogged while in office, but a running joke was that he couldn't jog past a McDonald's.
After leaving office, he followed the South Beach diet and lost weight. In 2004 he underwent coronary bypass surgery.
George W. Bush, 54 when he became president in 2001, had admitted to drinking problems earlier in his life, but had said he quit drinking in 1986 upon turning 40. His doctors said in 2001 that he ran three miles four times a week and lifted weights. His treadmill tests, blood pressure and cholesterol levels were excellent.
Such scrutiny hasn't always been the case. In fact, presidents and presidential candidates throughout history have manipulated or omitted key facts regarding major health ailments.
When President Grover Cleveland had cancer surgery of the jaw in 1893, the surgery took place on his yacht to keep it quiet.
When former Sen. Paul Tsongas sought the Democratic presidential nomination in 1992, he and two of his doctors said he had been cancer-free since a bone marrow transplant in 1986. He put on a Speedo and swam laps for the press. Later in the campaign, however, he said he had been treated for a recurrence of lymphoma in 1987. He ultimately withdrew from the race and died in 1997.
Perhaps the biggest health issues have been associated with Franklin Delano Roosevelt. When he ran for a fourth term in 1944, the public was led to believe his health was excellent. Yet he had a history of hypertension and cardiac problems.
He died on April 12, 1945, of a cerebral hemorrhage, only 82 days into his fourth term. He was 63.
''His blood pressure was off the charts, and they would tell people it was perfectly normal,'' says Jean Edward Smith, presidential historian at Marshall University and author of FDR (Random House, 2007).
Roosevelt also had polio. Yet newspapers did not publish pictures of FDR in a wheelchair; newsreels did not show him walking with braces.
''In part it was wartime concern not to show a weak president. In part it was that the press found it irrelevant,'' Smith says.
Many were shocked when FDR died. Vice President Harry Truman ascended to the presidency during World War II. ''When Truman was caught so unprepared, people began to focus on the issue,'' Smith says.
After Truman, Gen. Dwight Eisenhower, who assumed office in 1953 at age 62, released only general medical details while campaigning. But after he was in the hospital for seven weeks in his first term from a heart attack, ceding power to Vice President Richard Nixon, he released detailed records of his treatment. He won a second term, during which he had a stroke.
John F. Kennedy, who in 1960 was the youngest person elected to the presidency at age 43 -- was afflicted with several maladies, most of which were covered up by his staff, family and the press, says Dallek, Kennedy's biographer. Americans generally knew of Kennedy's World War II exploits aboard his Navy patrol boat, PT-109, which was rammed by a Japanese destroyer near the Solomon Islands, exacerbating his already troubled back. But they didn't know the extent of his back condition.
Kennedy also suffered from Addison's disease (a malady of the adrenal gland), colitis and intestinal infections, Dallek's book says. Dallek was given access to the president's medical files by his family.
Today, candidates understand they must be open about their health.
McCain uses humor, joking about being ''older than dirt.'' He and his staffers point to his mother, Roberta McCain, 96, who was sitting in the audience at the recent GOP convention. They say less about his father, who died at age 70, and his paternal grandfather, who died of a heart attack at 61.
Obama talks about the difficulty of quitting smoking. He says he hasn't smoked in months, joking that fear of his wife was his main motivation to quit.
Still, it wasn't a physical problem but a mental health issue that led to more aggressive reporting on the presidential candidates' medical conditions, says Greenberg, the Nixon biographer.
In the 1960s, Eagleton had checked into the hospital three times for treatment of depression, twice receiving electroshock therapy. He didn't tell the public -- or McGovern. When the news was leaked to the press, Eagleton resigned from the ticket.
"That was when we got into the question of, 'What else are they hiding from us?' '' Greenberg says.
The medical scrutiny extends to candidates' wives. Cindy McCain acknowledges she was addicted to prescription pain killers. Betty Ford, after her husband, Gerald, had left office, said in 1978 she had been addicted to pain killers and alcohol. Kitty Dukakis, whose husband, Michael, lost the presidential election to the elder George Bush in 1988, acknowledged she had been treated for amphetamine addiction in 1982. She said this in 1987 during her husband's campaign.
Releasing medical records has become essential. Jimmy Carter had to overrule his own doctor to get his records released in 1976. The doctor had balked, saying he didn't want to damage the principle of doctor-patient confidentiality.
Says Dallek, the presidential historian: "I think the line should be drawn on the side of privacy, except when someone is running for president of the United States -- with a finger on the nuclear trigger.
"Then we're entitled to know the full story of their health.''