When Mitt Romney's aides told him last month that the main reason they were skipping Sen. Jim DeMint's Labor Day forum was that he had already committed to attend an event in New Hampshire that day, the candidate was not pleased.
"So you're telling me the reason we're not doing this is logistical?" Romney asked, according to one aide. "That's not good. We're going to the DeMint forum. Make it so."
In addition to the New Hampshire stop, Romney had a flight scheduled later on Labor Day to Nevada for the, and aides were leery of overloading him with an event in another corner of the country -- and in South Carolina, no less, which has not figured significantly in his campaign's strategy.
But Mitt Romney has never been the kind of candidate who's especially concerned about the negative repercussions of doing too much.
And so just after the sun rose on Sept. 5, Romney's bags were already waiting on the front step of his Belmont, Mass., home when director of operations Will Ritter arrived to pick him up. It was 6:45 a.m. His day would end at 11 p.m. in Colorado, since the private campaign jet did not have enough fuel to make it all the way to Nevada without a layover.
Presidential campaigns are not endeavors that treat laziness kindly, and all of the Republican contenders have packed and demanding schedules.
But over the course of his last White House run and through the first few months of this one, Romney has proven himself to be a particularly hardworking campaigner who never seems to lose focus, no matter the hour of the day. This tirelessness is a potentially significant asset that could pay major dividends as the candidates' already taxing days become even more arduous.
"He's the kind of guy who will see a hole in the schedule and instead of thinking 'long lunch,' he thinks of doing a campaign headquarters stop-by or a radio interview," Ritter, who has been a fixture at Romney's side since 2006, told RCP. "He can't be stopped, and you almost have to trick him into not doing as much."
Romney is often criticized for lacking an ease with everyday interactions, a quality that has made Rick Perry such a formidable retail campaigner. While many Romney aides dispute the notion that he doesn't relate well to voters, there is little doubt that connecting on a human level is not one of his stronger assets.
But Romney has an often overlooked ability to impress voters simply by being "on" at every moment. In this sense, being a bit robotic can have its advantages.
"I still marvel at the energy he has," said Kevin Madden, Romney's 2008 national press secretary who remains in frequent contact with the candidate's top aides. "When I worked on that campaign in 2008, I was 34 years old and I couldn't keep up with him. I would need half a pot of coffee in the morning, and I was dragging by night. I lost my temper once or twice a day, but I saw him lose his maybe once or twice throughout the whole campaign."
A hard-charger throughout his life in business and during his four political campaigns, the effects of Romney's clean-living lifestyle are self-evident. As one aide put it, "He's not the guy who gets a beer at the end of the night, so you save that hour-and-a-half."
During his 2008 presidential run, it was not unusual for Romney to regale a breakfast crowd in Iowa with an anecdote about his run through the local neighborhood before the early-morning event began or for him to leave aides scrambling to keep up as he jogged through another New Hampshire parade.
Romney is not immune from the allure of the fast-food joints that are staples of just about any campaign-trail diet, but he peels the skin off of his Kentucky Fried Chicken before eating it and orders the turkey burger or grilled chicken sandwich when it's on the menu.
He may be 64 years old, but Romney looks and moves like someone much younger. And while he has committed his fair share of campaign-trail gaffes, he has never appeared to do so out of exhaustion.
In an extended campaign in which every day becomes more physically and emotionally exhausting than the last, his endurance could provide a significant advantage over the rest of the GOP field, all but one of whom are first-time national candidates.
"It's clear that he's in tremendous shape, and there's something to be said for that because this is a marathon," said New Hampshire GOP operative Rich Killion, who worked for Romney in 2008 but is currently unaligned with any candidate. "But it's a marathon predicated on combining six months of daily sprints, and he seems to have the makeup for it. That, coupled with the past experience of having gone through it before, is only going to help him."
As part of their early efforts to prevent Romney from being overexposed, his aides in Boston have for months limited his media appearances and public events, in stark contrast to this point four years ago, when he had been going full-throttle for over six months.
But as the campaign enters the pivotal fall season, no one will be more prepared for the grueling pace than the former Massachusetts governor.
Romney's New Hampshire consultant Jim Merrill noted that the candidate has already engaged directly with voters at 10 town-hall meetings in the state, while other contenders have thus far relied more on quick meet-and-greets and scripted speeches.
"It's not only the amount of events he does, but it's the nature of events he does, which are demanding," Merrill said. "He's doing that and no one else is doing it. I think it's a testament not only to his character but his stamina. It's Vince Lombardi football -- three-and-a-half yards in a cloud of dust -- and that's what he's doing up here and will continue to do."
Scott Conroy is RealClearPolitics national political reporter and CBS News contributor.