BEDFORD, N.H. — An ascendant Marco Rubio was knocked off stride in a Republican debate Saturday night, at a critical moment just days before the high-stakes New Hampshire primary with many of the state’s voters still undecided.

New Jersey Gov. Chris Christie attacked Rubio and attacked him hard, leaving Florida’s freshman senator grasping for answers. Although Rubio bounced back in the second half of the debate, that early fumble left a deep impression.

After Ted Cruz and Donald Trump finished one-two in the Iowa caucuses last week — with Rubio a surging third — the Republican presidential candidates who lagged behind made no secret that they would arrive at the ABC News debate here with knives sharpened. Christie and Jeb Bush, in particular, have heightened their attacks on Rubio in recent days, as both face the prospect of a last stand in New Hampshire.

On the campaign trail, with the winds of favorable polling at his back, Rubio had seemed to dismiss these barbs with ease. But, once on stage here, the dynamic flipped, with Christie doing most of the dirty work.

In a pivotal exchange, he asserted that Rubio “simply does not have the experience to be president of the United States” and that nominating him would repeat the mistake of choosing Barack Obama, a first-term senator when he ran for the White House.

Rubio responded, “Let’s dispel with this fiction once and for all that Barack Obama doesn’t know what he’s doing.” What Rubio seemed to be implying is that, for conservatives, the real problem with the current president isn’t the level of experience he brought to the job, but how liberal he’s turned out to be as president — and how effective he’s been at pushing his agenda.

But he didn’t say it quite that way and Christie jumped on Rubio’s remarks as a canned talking point — and Rubio countered with the same line once more. “There it is, the memorized 25-second speech,” Christie interjected. Some members of the crowd appeared to boo Rubio’s repetition and the young senator came across as flustered.

“He gets very unruly when he gets off his talking points,” Christie said. But an undeterred Rubio returned to his point, using the same line once more later in the evening.

Bush piled on, but more subtly than Christie, calling Rubio “a gifted politician.” At 44, Rubio is the youngest candidate in the field. And as he fought off these attacks, Rubio looked callow: Unfortunate staging placed him squarely between Bush and Donald Trump, men in their 60s, both of whom towered over him physically.

Rubio spokesperson Alex Conant tweeted that the senator dominated the debate by other measures, including as the most-searched candidate online while on stage. His campaign manager, Terry Sullivan, tweeted that Rubio raised three times as much money during this debate as he did during any other.

“What Gov. Christie was trying to do was to knock Marco out, to kill him dead,” Rubio strategist Todd Harris told reporters in the spin room following the debate. “He took his best shot, and he failed.”

But Christie’s campaign operatives pushed an alternate interpretation: that the debate altered the course of the race in New Hampshire. “The rush to coronate Marco Rubio as the nominee is off,” Senior Strategist Mike DuHaime told reporters afterward. “I think people are hitting pause on that and are taking a second look at Christie, especially, and will probably take a second look at the whole field.”

Whether Rubio will be able to maintain his strong standing in New Hampshire is an existential question for Christie, Bush and Ohio Gov. John Kasich, all of whom are betting their candidacies on placing in the first-in-the-nation primary.

Meanwhile, the current New Hampshire frontrunner escaped the harsh scrutiny Saturday faced by Rubio. Donald Trump has held a commanding lead in the polls for months here. And although Rubio’s stumbles could help the cause of the three governors running against him, each hoping to untangle himself from the others and emerge as a clear choice in the so-called establishment lane, they did little to blunt Trump’s standing.

The billionaire businessman, who has been appealing to the state’s populist sensibilities, is leading his closest rival by nearly 15 points in the RealClearPolitics polling average. But his loss in Iowa after a series of polls projected him as the winner there raises questions about his ability to close the deal when voters actually cast their ballots. Strategists and surrogates from rival campaigns claim Trump’s lead is narrower than polls suggest, and point to the results in Iowa as an example of the potential of strong campaigns with robust ground operations to defy expectations.

Trump had to cancel a town-hall event on Friday after snow grounded the private plane on which he travels to and from the state. Trump has been criticized for flying in and out of the early states without spending the night, eschewing the retail type of politicking so integral to presidential campaigns here. He was in the state on Thursday, for example, and flew back to New York before attempting to return to New Hampshire the next day. He held a rally Friday night in South Carolina, which holds its GOP primary Feb. 20.

Still, Trump has shown signs of improving as a candidate after coming second in Iowa. He added events to his relatively light -- compared to his rivals -- New Hampshire schedule to include more traditional and intimate kinds of stops. He has five more events on the calendar here between now and Monday.

The clearest signs of Trump’s evolution came Saturday night on the debate stage. He struck at Cruz, Bush, and Rubio but was otherwise more subdued than usual. He was also helped at times by some rivals taking a pass when invited to strike at him.

Cruz railed against Trump throughout the week, questioning his temperament and characterizing him as immature and unfit to be in charge of the country’s nuclear arsenal. When asked about his assessment Saturday night, however, the Texas senator declined to engage. “I think that is an assessment the voters are going to make,” he said.

Trump did not return the favor. “I respect what Ted just said, but if you noticed, he didn't answer your question,” he said. “That’s what's going to happen with our enemies and the people we compete against.”

Rubio also declined to attack Trump when asked whether he still believes the businessman isn’t running as a conservative. Instead, he pointed to his own definition of conservatism, and acknowledged that “Donald touched on one of them.”

Trump couldn’t hold back in the end, though, using part of his closing statement to question Cruz’s Iowa win.

For his part, Cruz apologized on stage to Ben Carson for his campaign telling supporters just before caucuses began in Iowa that the retired neurosurgeon was leaving the race. “It gives us a very good example of certain types of Washington ethics,” Carson said.

Cruz doesn’t need to win New Hampshire, but a strong showing here, augmented by his caucus win, would further propel him into the Southern state primaries.

These tense exchanges followed an inauspicious start to the debate. As the moderators introduced the candidates, their signal to walk on stage, Ben Carson missed his cue. Instead of walking to his podium, he stood awkwardly off stage, but on camera, even as other candidates passed him by. Trump, when his name was called, stayed back with Carson. It took a few painful minutes, and emphatic gestures by a producer, to finally get all of the contenders in position.

But the night will likely be remembered for Rubio’s stumbles, which reinforced his characterization by rivals as a robotic product of Washington with little concrete experience.

A misstep by freshly minted Rubio supporter Rick Santorum earlier this week only helped to seed this narrative: Asked by host Joe Scarborough on “Morning Joe” to name one accomplishment of Rubio’s, Santorum came up empty.

“If you look at being in the minority in the U.S. Senate in a year when nothing got … four years where nothing got done, I guess it’s hard to say there are accomplishments,” the former presidential candidate finally said. Bush’s campaign turned the damaging moment into an anti-Rubio attack ad.

Most detrimental to Rubio, the attacks come at a time when voters are particularly impressionable. New Hampshire has historically been a volatile, late-deciding primary state, in which candidates’ standing can swing wildly during the final days before voting. During the 1980 Republican primary, the catalyst for such a swing was a moment in a debate between Ronald Reagan and George H.W. Bush.

Rubio’s rivals see a similar inflection point in Saturday’s debate. In the spin room afterward, aides for Christie and Bush predicted undecided voters would fall in their favor.

Rebecca Berg is a national political reporter for RealClearPolitics. She can be reached at rberg@realclearpolitics.com.