MANCHESTER, N.H. >> Frustrated and flailing as his candidacy threatens to slip away, Gov. Chris Christie of New Jersey is embarking on a scalding effort over the next week to discredit Sen. Marco Rubio of Florida, the man he blames for undermining his campaign and whose ascendancy he deeply resents.

And Christie has a secret ally: Jeb Bush.

Christie, whose White House hopes hinge on a strong showing in New Hampshire, is unleashing the kind of cutting and personal attacks that brought him renown in New Jersey but that pose a far greater risk in a presidential campaign, where they can harm the assailant as much as the intended target.

Over the past 48 hours, Christie has mocked Rubio as a cosseted “boy in the bubble,” derided him as “constantly scripted,” likened him to “the king of England,” and, perhaps most creatively, compared his Senate career to that of a helpless fourth-grader who is told which chair to sit in at school.

On Wednesday, Christie challenged anyone “to show me the significant accomplishment that Senator Rubio has done while he’s in the United States Senate.”

“I can’t find one,” Christie added.

For Christie, the verbal barrage is a carefully calibrated attempt to damage Rubio’s standing in New Hampshire before Tuesday’s primary — and to set up a showdown with him in Saturday night’s Republican debate, possibly Christie’s best chance to impede Rubio’s surging candidacy.

Rubio’s strong third-place performance in Iowa on Monday night, and his steady improvement in the polls days before the voting there, alarmed Christie and his mainstream Republican rivals, including Bush, who fear Rubio’s emerging as a breakout figure in New Hampshire.

The shared concern has even prompted the opening of a back channel: Members of the Bush and Christie campaigns have communicated about their mutual desire to halt Rubio’s rise in the polls, according to Republican operatives familiar with the conversations.

While emails, texts and phone calls between operatives in rival campaigns are not uncommon in the tight-knit world of political strategists, the contact between senior aides in the two campaigns has drifted toward musings about what can be done to stop or at least slow Rubio, the operatives said.

In a sign of a budding alliance, the aides have, for example, exchanged news articles that raise potential areas of vulnerability for Rubio. There is no formal coordination, the operatives stressed, but rather a recognition of a shared agenda.

“We do have similar goals,” an adviser to Christie said.

For their part, Christie and Bush are finding ways to praise one another in public and cozy up to each other in private: Bush telephoned Christie on Monday to wish him good luck in the caucuses. (Christie was not available and the call went to voice mail.)

During a CNN interview on Wednesday, Bush put their bond on display for all to see. “I love Chris Christie,” said Bush, the former governor of Florida. “He is a — he’s a great campaigner, he’s a good friend and he’s been an effective governor.”

A division of labor seems to have taken hold. While a well-financed super PAC supporting Bush assails Rubio on television and in the mail (it will release a new batch of ads Thursday), Christie has stepped up the critiques on the campaign trail.

“Jeb can’t do that sort of stuff,” said an adviser to Christie, referring to the New Jersey governor’s slashing “boy in the bubble” attack on Rubio and his comfort with political street fighting. “They don’t have the weapon.”

The alliance makes sense, operatives said. A strong primary night in New Hampshire for Rubio would allow him to make a compelling case to voters and donors that he is the surest bet for unifying the party against its most divisive presidential contenders, Donald Trump and Sen. Ted Cruz of Texas. And it would make it harder for Bush and Christie to justify remaining in the race.

As the primary nears, Christie is expressing disapproval for Rubio in increasingly elaborate and colorful ways.

At Londonderry High School, he found a fourth-grader named Matthew in the audience and posed a series of questions to him about his daily routine.

Did he have a set time when he had to be at school? Yes, the boy said.

An assigned desk to sit in? The boy nodded.

A list of questions to answer each day — and a summer break? Matthew answered affirmatively.

Christie pounced: “They do that in the United States Senate, too!” The audience roared.

The Senate, he said, was a foolhardy place from which to pick a president.

“What we are all wondering is: How does that train you to be president?” Christie said. “Because that’s not the way the presidency works, and it is certainly not the way the governorship works.”

Joe Pounder, a spokesman for Rubio, dismissed Christie’s broadsides, pointing to a record that he suggested was outside the Republican mainstream. “No amount of hot air or made-up facts can distract from Chris Christie’s liberal record,” he said.

For Christie, there is a personal dimension to the onslaught against Rubio. After Christie started to gain in the polls in New Hampshire late last year, allies of Rubio paid for a ferocious series of television ads and direct-mail pieces attacking the New Jersey governor for deviations from conservative orthodoxy and over the gridlock-creating lane closures on the George Washington Bridge overseen by his allies.

In a final flourish, they showed images of Christie together with President Barack Obama, evoking a divisive moment after Hurricane Sandy when the governor praised and warmly greeted the president.

Beyond that, Christie evidently feels a sense of betrayal over the inroads that Rubio has made with major Republican donors, particularly the New York-area financiers whom Christie spent years courting.

“So he got a couple of donors,” Christie sniffed, adding, “Good for him.”

© 2016 The New York Times Company