Republican presidential candidate Jeb Bush addresses a gathering during a campaign stop outside Geno’s Chowder and Sandwich Shop Thursday in Portsmouth, N.H. (Charles Krupa/AP)

Jeb Bush’s presidential campaign was at an inflection point Thursday as the candidate faced a choice of sticking with his long-term strategy or yielding to criticism from supporters and senior Republicans who are demanding fundamental changes to his sputtering candidacy.

Significant concerns about Bush’s performance were magnified on the debate stage here Wednesday night, and the former Florida governor awoke to his harshest criticism yet. Bush faces the real possibility that a substantial amount of money and momentum will move to Sen. Marco Rubio (Fla.) as the party’s mainstream conservative wing’s favored candidate unless he moves quickly to rebalance his candidacy.

“I could have done better,” Bush conceded on a conference call Thursday afternoon with top donors and state chairs, according to a person who participated.

His debilitating performance was for many allies a cause for alarm. He sowed serious doubts about his ability to effectively prosecute the case against not only his Republican rivals but also the potential Democratic nominee, Hillary Rodham Clinton, in what is expected to be a brutal general-election campaign.

Interviews Thursday with strategists and fundraisers throughout the Republican firmament underscored that there are no particularly attractive options for Bush to breathe new life into his campaign. But top Bush campaign officials said they would not be panicked by what one dismissively called “the insanity of pundit world.”

The Fix's Chris Cillizza explains how Republican presidential candidate Jeb Bush missed his opportunity and opponent Marco Rubio had a strong performance in the third GOP debate. (Zoeann Murphy/The Washington Post)

Sig Rogich, a longtime Bush family adviser who now serves as the former governor’s finance chairman in Nevada, said Bush was visibly uncomfortable going on the attack. But Rogich expressed confidence that the candidate can show the toughness that is required.

“His gentlemanly manner got in the way of tough, gnarly debate activity. Everyone realizes it’s not a cream-puff situation up there,” Rogich said. “He understands where the campaign is right now, and I think he’ll act accordingly.”

Tim Pawlenty, a former Minnesota governor, put it slightly differently in an interview on CNN that echoed the sentiments of many Republicans outside the Bush campaign. “He’s a meritorious candidate, but he’s underperformed so far,” Pawlenty said. “. . . He’s going to have to up his game, or the marketplace is going to move away from him.”

Returning to the campaign trail Thursday afternoon in Portsmouth, N.H., Bush brushed aside the most dire predictions. “It’s not on life support,” he told reporters. “The end is not near. Memo to file.”

On the stump, beneath a banner that read “Jeb Can Fix It,” Bush drew a contrast between what it takes to perform in debates and what it takes to be president. “I wish I could talk as well as some of the people on the stage, the big personalities on the stage,” he said, “but I’m a doer.”

Playing at the Portsmouth event was “The Workingman’s Hymn,” which has this oft- repeated refrain: “I know that we can turn it around.”

As they absorbed the criticism on Thursday, top Bush advisers were singing the same tune.

“Jeb is often underestimated as a competitor,” said Sally Bradshaw, Bush’s closest adviser. “He’s in a time of testing right now. My guess is he shakes himself off and gets right back on the field. In fact, that’s what he’s doing right now, today in New Hampshire.”

In Wednesday’s CNBC debate, Bush lunged at Rubio, his onetime protégé, accusing him of ducking his responsibilities as a senator to feed his ambition to become president. But Rubio so smoothly countered the attack that Bush was left looking weak and ineffective.

Given the buildup about the importance of the debate to Bush, coming only days after he had announced deep budget cuts and a retrenchment, his generally ­listless performance deepened doubts at a moment when he needed to offer reassurance.

Bush’s loyal supporters, including some longtime friends, were aghast at the candidate they saw on the television screens. Gone, they said, was the optimistic message of economic empowerment that was to be the foundation of his campaign. Gone, too, was the confident competitor who dominated Florida politics during his two terms as governor.

“When I see that debate, I don’t know who that man is,” Ana Navarro said on CNN. “That’s not the guy I’ve known for 25 years.”

One Bush fundraiser, who like others spoke on the condition of anonymity to be candid, said the attack on Rubio “was just not the image and the right tone of the person whose campaign is built on ‘the right to rise,’ not the right to step on your protégé to get to the top.”

Rick Wilson, a Florida-based strategist unaffiliated with a candidate, said: “For weeks, the money guys were with Jeb. Last night, those people were telling me, ‘I’m calling Marco’ or ‘I’m done’ or ‘That was absurd’ or ‘I’m finished.’ ”

But some in the Bush operation insisted that over time the attributes that Bush brings to the fore will outweigh the weaknesses exhibited on Wednesday night.

“He’s not going to be the best debater, but he is the adult up there,” campaign adviser Austin Barbour said. “He is the most prepared to be president. He’s not a risk at this time.”

Bush hopes to highlight those attributes next week when he gives what aides are billing as a major speech in Tampa to coincide with the publication of his book, “Reply All,” a collection of the e-mail messages he sent as governor.

Previewing the Tampa address, spokesman Tim Miller said, “It’s going to look at how we can overcome the pessimism and the decline of the Obama era, and what needs to be done, and what kind of leadership skills are required, and who is the person that’s capable of doing it.”

From Tampa, Bush will embark on a week-long campaign trip showcasing personal reflections of his colleagues and constituents in Florida, ending with a bus tour of the first primary state of New Hampshire.

Bush advisers said they still believed highlighting the record he compiled as governor between 1999 and 2007 would eventually pay dividends. “These messages, these stories, they do take time to sink in,” Miller said. “It requires repetition. It requires continuing to make the case and being dogged about it.”

His Florida record also has been the subject of millions of dollars in television advertisements in the early states by his well-funded super PAC, Right to Rise. Bush advisers said the ads had helped improve his image and his advantage on issues such as the economy, particularly in New Hampshire.

“This is a long process with many more debates,” said Paul Lindsay, a spokesman for the super PAC. “As Republican voters vet each candidate, we are confident they will ultimately nominate Jeb Bush as the best-prepared candidate to lead our party with his proven policy heft and real record of conservative accomplishment.”

But the ads have not produced an uptick in his overall support in public polls, and outside the campaign, Republican operatives said they had seen no evidence that the Florida focus was resonating in any significant way. They also said it made Bush seem fixated on the past at a time when voters are concerned about the future and when Rubio and other candidates are sounding forward-looking themes.

For Bush to rise, he will need to peel supporters away from the top two candidates, Donald Trump and Ben Carson, but he appears badly positioned to do so relative to Rubio and Sen. Ted Cruz of Texas. Supporters of Trump and Carson generally have a negative view of Bush and a positive view of Rubio, strategist Matthew Dowd said.

“Jeb has no reservoir of positive support,” Dowd said.

Bush’s presumed ace in the hole is the super PAC, which has raised more than $100 million. Other candidates had long assumed much of that money would be used for negative advertising. But as one senior operative put it, “If Governor Bush can’t land a punch on Marco Rubio on the debate stage, then the super PAC can’t do it from the peanut gallery.”

Even critics of the strategy said that the most serious problem that needs fixing is the candidate himself. Republicans interviewed laid out several unpalatable choices. One would call for Bush to do what Sen. John McCain (R-Ariz.) did at this stage in the 2008 race when he was at a low ebb and retreated to a one-state strategy in New Hampshire.

Another is to turn completely away from attacking Rubio or billionaire candidate Trump and become the “joyful” warrior that he had vowed to be. But in a year in which Republicans are measuring their candidates through the prism of who can take on Clinton, being joyful might not be enough.

“The central mission that Bush laid out at the beginning of this campaign about running as a ‘joyful’ warrior and talking about where he wants to take the country and being very positive and being his own man energized a lot of folks,” said Kevin Madden, a veteran of past presidential campaigns. “The challenge right now is that the campaign he’s being forced to run runs somewhat counter to that ethos.”

Bush advisers say their candidate is still in a position to draw the most effective contrast with Clinton. “Hillary has nothing to show for her career in government. We need to nominate someone with real accomplishments, who can draw votes from places Republicans have struggled lately,” Bush national finance co-chairman Jack Oliver said in a message. “Jeb is that guy.”

Still, one prominent Bush donor from the Midwest wrote in an e-mail: “I just don’t see the fire in the belly that he needs to move through our brutal primary battles and the ultimate Deathmatch with the Clinton machine. I don’t see it!”

O’Keefe reported from Washington. Sean Sullivan and Tom Hamburger in Washington and Anne Gearan in Portsmouth, N.H., contributed to this report.