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When the “super PAC” backing Senator Marco Rubio assailed Gov. Chris Christie’s record in New Jersey in tough new television ads this week, it was as if Mr. Christie had been presented with a gift.

Mr. Rubio is deft at delivering speeches. But nobody in the Republican presidential field, with the exception of Donald J. Trump, is as skilled as Mr. Christie at belittling his rivals.

And when Mr. Christie slashed back, he aimed for Mr. Rubio’s biggest vulnerability — his relative youth and inexperience — telling the conservative radio host Laura Ingraham that Hillary Clinton would pat Mr. Rubio “on the head and then cut his heart out” if they squared off in a general election.

“This isn’t my first rodeo,” said Mr. Christie, a former federal prosecutor. Referring to Mr. Rubio, he added: “This guy’s been spoon-fed every victory he’s ever had in his life.”

On Thursday in Nashua, N.H., Mr. Rubio laughed off Mr. Christie’s barbs and underscored his argument that Mr. Christie was a fake conservative who would extend the Obama administration, not undo its work. But the back-and-forth was opportunity-rich for Mr. Christie.

It gave him a chance to grab the spotlight after months of being eclipsed by Mr. Trump as the bluntest-speaking candidate in the race. It gave him a chance to showcase his talents as a political performer. And it gave him a chance to demonstrate that Mr. Rubio, 44, a first-term senator from Florida, might not be as battle-ready as his supporters believe, or as tough as his muscular new campaign ads might suggest.

The intensifying conflict came amid a fluid contest in New Hampshire, where Mr. Trump has led in the polls for months, but where surveys show that Mr. Rubio and Mr. Christie have been on the rise. The two are vying for second or third place, essentially, with two other candidates — Jeb Bush, the former Florida governor, and Gov. John R. Kasich of Ohio — who are engaged in a game of political leapfrog.

Increasingly, as he did in mocking Mr. Rubio, Mr. Christie has been arguing to voters that he is electable in November — and that, by implication, his rivals are not.

Mr. Bush, who the polls show is also inching up in New Hampshire, is also engaging in increasingly aggressive combat. A piece of mail from the super PAC supporting him, Right to Rise USA, arrived in homes in New Hampshire on Thursday: “Is Chris Christie a fiscal conservative? Not exactly,” it warns, alongside a litany of red-letter warnings about Mr. Christie’s economic stewardship of New Jersey. The group is also running television advertisements attacking Mr. Rubio in several states.

Mr. Christie’s tarring of Mr. Rubio — whom he accused of starting the fight — came as his television ads preach Republican unity for the sake of defeating Mrs. Clinton.

“What’s amazing about the Christie campaign is that what he says on the campaign stump totally contradicts what he’s been saying in his paid television advertising, which shows he doesn’t have a strategy,” said Scott Reed, a longtime Republican strategist who is the top political adviser to the U.S. Chamber of Commerce.

For Mr. Rubio, the exchange was the second he has had lately with a rival who hit back harder.

The first was Senator Ted Cruz, who used a debate over who was more open to “amnesty” in overhauling the nation’s immigration to repeatedly tie Mr. Rubio to Senator Chuck Schumer, the New York Democrat who is loathed by many conservatives.

Moreover, while Mr. Rubio’s campaign worked to discredit Mr. Cruz for his votes against military spending and domestic surveillance, Mr. Christie was gaining steam in New Hampshire.

He began taunting Mr. Rubio for what he said was shirking his Senate duties. “Dude, show up to work and vote,” the governor said late last month. He also suggested Mr. Rubio was uninterested in campaigning, saying, “He’s never here.”

Mr. Rubio did not hit back hard at Mr. Christie until Thursday, when he delivered a detailed attack on Mr. Christie’s record.

“Chris has a very liberal record for a Republican,” Mr. Rubio told reporters in Nashua. “I mean, he supported Common Core. He ran for office as a supporter of gun control. He personally gave a contribution to Planned Parenthood. So I’m sure he doesn’t really want to have a conversation about the issues, because the truth is our next president has to be someone who’s going to overturn all the damage Barack Obama has done to America, not continue it.”

Yet the sniping Mr. Rubio is now engaged in is somewhat at odds with his professed image as an Obama-esque Republican: upbeat, optimistic and above the fray. Mr. Rubio also has noticeably darkened his tone on the campaign trail. Once the candidate who dismissively criticized Mr. Trump’s “Make America Great Again” slogan by telling audiences, “America is great,” Mr. Rubio has been lamenting the country’s trajectory more and more in recent days.

“America is a great nation in decline,” he said on Thursday at a house party in Bedford, N.H.

Voters seemed confused by that message.

Debbie Urbanik, 48, of Dunbarton, N.H., said she was surprised by how hard a line Mr. Rubio took on Mr. Obama. She said she voted for the president, but could see herself voting for a Republican this year. She was not sure yet whether that might be Mr. Rubio.

“To listen to somebody stand up there saying, ‘This is all Barack Obama’s fault,’ is not a good message,” she said. “If I had heard him stand up there and say, ‘Listen, our economy is broken and it started with this, and it evolved through this, and I want to fix it this way,’ I would have gotten excited.”

“I was hoping I would walk out of here like, ‘Yeah,’” Ms. Urbanik added. “And I don’t feel as great as I wanted to. It was so much of a blame game.”

Jonathan Martin contributed reporting.