Speaking to donors at a San Diego fund-raiser last month, Barack Obama reassured the crowd that he wouldn’t give in to Republican tactics to throw his candidacy off track.

“Listen, I’m skinny but I’m tough,” Sen. Obama said.

But in a nation in which 66% of the voting-age population is overweight and 32% is obese, could Sen. Obama’s skinniness be a liability? Despite his visits to waffle houses, ice-cream parlors and greasy-spoon diners around the country, his slim physique just might have some Americans wondering whether he is truly like them.

But too much time in the gym can cause problems, as Sen. Obama learned last month after he made three stops to local Chicago gyms in one day, for a total of 188 minutes. The marathon workout session sparked a widely circulated Associated Press article titled “Obama Becomes a Gym Rat.” In it, the reporter wrote, “Sometimes it’s hard to tell if Barack Obama is running for president of the United States or Mr. Universe.”

Republicans have recently picked up on the senator’s fitness regimen. On Wednesday, the McCain campaign launched a new ad titled “Celeb” that compares Sen. Obama to Paris Hilton and Britney Spears. In a memo to reporters explaining the ad, McCain campaign manager Rick Davis wrote, “Only celebrities like Barack Obama go to the gym three times a day.”

President George W. Bush rode with cycling superstar Lance Armstrong on Saturday, August 20, 2005.

The seven-time Tour de France champion joined the president for a two-hour, 17-mile trek through the canyons and river-crossings of Bush's 1,600-acre Crawford, Texas ranch. The two only rested for ten minutes during the ride.

George W. Bush, 59, took up mountain biking after a knee injury forced him to give up jogging a couple of years ago.

He's described by his doctors in his annual physical as being in "superior" condition for a man his age.

He participates in a six-day-a-week workout regimen and showed his heart rate monitor to reporters who rode with him. The monitor showed he burned 1,493 calories in the prior 17-mile ride that also lasted two hours.

Secretary of State Condoleezza Rice gets up at 4:30 in the morning to exercise. She set up a mini gym in her apartment with a stationary bicycle and an elliptical machine.

President Bush gets up early, too, is in the Oval Office by 7 a.m. and works out at midday. The president used to run a 7-minute mile but started mountain biking a couple of years ago to give his knees a break.

He's such an inveterate exerciser that he has little patience for anyone who doesn't find time. Bush says if he can find time to work out, anyone can.

Given the importance of his job, it is astonishing how much time Bush has to exercise. His full schedule is not publicly available. The few peeks we get at Bush's daily routine usually come when some sort of disaster prods the White House Press Office to reveal what the president was doing "at the time."

Earlier this year, an airplane wandered into restricted Washington, D.C., air space. Bush, we learned, was bicycling in Maryland. In 2001, a gunman fired shots at the White House. Bush was inside exercising. When planes struck the World Trade Center in 2001, Bush was reading to schoolchildren, but that morning he had gone for a long run with a reporter. Either this is a series of coincidences or Bush spends an enormous amount of time working out.

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In 2002, Bush fired Lawrence Lindsey, his overweight economic adviser. Lindsey's main crime was admitting to Congress that the Iraq war might cost $200 billion, at a time when the administration was trying to cut taxes and was insisting that the war would cost nothing. But compounding things was the fact that, as The Washington Post reported, Bush "complained privately about (Lindsey's) failure to exercise."