MANCHESTER, N.H. — Millions of people watched Marco Rubio’s televised tailspin in the opening minutes of last weekend's Republican presidential debate — but what, exactly, they saw depended on the viewer.

To rivals, Rubio’s reflexive retreat to the same snippet of well-rehearsed rhetoric — over and over, and over, and over again — was proof of the freshman senator’s status as a lightweight. To supporters, the wobbly display was a forgivable fluke, one bad moment blown wildly out of proportion by a bloodthirsty press corps.

But to those who have known him longest, Rubio’s flustered performance Saturday night fit perfectly with an all-too-familiar strain of his personality, one that his handlers and image-makers have labored for years to keep out of public view. Though generally seen as cool-headed and quick on his feet, Rubio is known to friends, allies, and advisers for a kind of incurable anxiousness — and an occasional propensity to panic in moments of crisis, both real and imagined.

This jittery restlessness has manifested itself throughout Rubio’s life, from high school football games in Miami to high-profile policy fights in Washington — and in some ways, it’s been the driving force in his rapid political rise.

When Rubio was nearing the end of his final term as Speaker of the Florida House in 2008, he invited a small circle of loyal donors, local activists, and friendly media figures to an intimate breakfast meeting at Miami’s Biltmore Hotel to help him plot his next move. The consensus at the table was that he should wait for the right statewide race to open up — attorney general, maybe, or even governor. But Rubio, feeling the familiar itch of the achievement junkie, was distressed by the prospect of patiently waiting around. The next race he could conceivably enter was for the Miami-Dade mayorship, and according to people familiar with the meeting, Rubio worked himself into a minor tizzy trying to convince his skeptical breakfast companions that he should run: What if his donors got poached while he was out of the spotlight? What if his supporters abandoned him? He could be finished in politics if he screwed this up!

As his voice betrayed a growing agitation, some at the table began exchanging sideways glances, perplexed by the spectacle and slightly embarrassed for Rubio. Finally, Ninoska Pérez Castellón, a popular local radio personality who frequently interviewed Rubio on air, felt it necessary to interject with some tough love.

“Marco!” she snapped. “You could be governor, or even in Congress! You don’t want to burn yourself as mayor of Dade County.” Slow down and stop worrying so much, she told him. “People aren’t going to forget you.”

As one of the breakfast attendees recalled of the scene, “He was just missing that sense of maturity you want.”

Rubio has spent much of this campaign season contending with questions about his experience, as pundits and voters argue over whether he possesses the requisite gravitas for the Oval Office. One reason is his age: At 44, he is the youngest candidate in the field. But Ted Cruz is only a few months older — and has spent even less time than Rubio in the Senate — and yet he rarely finds himself fending off questions about his youth.

Another source of doubt may simply be Rubio’s appearance: Campaigning in middle-school cafeterias and gymnasiums across the Granite State this past week, Rubio’s carefully parted hair, lint-rolled quarter-zip sweaters, and stubble-free baby face give him the general appearance of a boy whose mother has dressed him up for church.

But more than age, record, or wardrobe, it is Rubio’s natural nervousness that makes him seem to so many who know him like he is swimming in his dad’s sport coat.

For all Rubio’s efforts at image control, he sometimes allows involuntary glimpses at his inner anxieties. For example, Rubio’s 2012 memoir, American Son, is — when read a certain way — less an inspiring tale of his unlikely rise to power, and more a harrowing chronicle of self-doubt and misery in the political arena. Indeed, for a politician defined by his sunny message and soaring rhetoric, Rubio’s 2010 Senate bid sounds, in his telling, like a merciless assault on his psyche: the race a gut-twisting roller coaster ride on which he was constantly convinced the next track-rattling twist, turn, or plummet would throw him from the cart and send him plunging to his death. The account is peppered with words like “inevitable humiliation,” "destined for failure," and “despair.”

From the moment the 2010 primary turned negative, the candidate needed a fainting couch every time an attack was lobbed his way, his aides recalled to me. It began when Charlie Crist’s campaign claimed Rubio had “tucked away” $800,000 in the state budget for new AstroTurf on the Miami-Dade fields where he played football. It was a relatively gentle jab as far as campaign combat goes, and the story quickly fizzled. But Rubio was positively wounded by the charge.

How can they say that? he mewed to his aides. They can really just make stuff up about me?

“I had never been in political combat like this, and [Crist’s] attacks stung me. ... It takes a while to get accustomed to [it],” he later wrote. Sure enough, as the race intensified, Rubio objected heatedly — and often emotionally — to each and every attack line unleashed by the Crist camp.

When a state senator who was backing the governor referred to Rubio as a “slick package from Miami,” he was aghast and ordered his aides to cry foul.

Dog whistle! Anti-Cuban! Racist!

When opponents accused Rubio of steering state funds toward Florida International University in exchange for a faculty job after he left office, he was indignant.

Outrageous! Slander!

On the stump, he continued to exude vigor and optimism — but inside, the accusations were tearing him up. “Because they were hurting me personally, I was certain these attacks were hurting our campaign,” he wrote in American Son. “I was sure they would blunt our momentum.”

They never did, of course, and in the official narrative put forth by Team Rubio, he emerged from the fight in 2010 stronger and more resilient than ever. But even on the night of his high-profile victory speech, he was second-guessing himself. After leaving the podium, he realized he’d forgotten to say a few words in Spanish — and for days, he struggled to accept compliments on the speech without noting this regret.

This hemming and hawing continued into his Senate career, especially after 2012 when high-powered political allies in Miami began lobbying him to join the Senate’s bipartisan push to overhaul immigration. Sympathetic to the cause but wary of the politics, Rubio went back and forth for weeks, talking through an endless loop of pros and cons, and agonizing over every potential pitfall. To his supporters at the Biltmore — a Miami political hub where he had long ago earned the diminutive nickname Marquito — there was little patience for his trademark tentativeness.

Ten valor, Marquito! came the refrain from the Biltmore chorus. Have courage!

“What’s the point of having political capital if you’re never going to use it?” Ana Navarro, a GOP commentator (and current Bush supporter), demanded during one early conversation with Rubio.

“He just lets these little things get to him, and he worries too much,” a Miami Republican complained after spending close to an hour sitting next to Rubio on a flight as he fretted over a mildly critical process story about him in the National Journal. “I’m just like, ‘Marco, calm down.’”

Even Jeb Bush Jr. expressed frustration with Rubio in an interview with BuzzFeed News late in 2012. “He’s got to actually execute and get something done, rather than just talking,” he grumbled.