(AP) -- New Jersey Gov. Chris Christie backed Donald Trump in the Republican race for president Friday, a powerhouse endorsement as the billionaire tries to beat back assaults on his character from a newly aggressive rival, Marco Rubio.

Christie joined Trump at a Forth Worth news conference and praised him as "the person who will go to Washington, D.C., and be able to absolutely turn the place around."

Christie said Trump was hands-down the better choice than Rubio, a first-term senator who "didn't show up for work."

"We don't need any more of these Washington, D.C. acts," said the governor, who had needled Rubio for missing Senate votes back when the governor was a candidate himself.

The shock endorsement came after Rubio, in an abrupt shift cheered by anxious Republicans nationwide, shattered months of relative peace with Trump in Thursday night's GOP debate. It marked the beginning of a long-awaited takedown effort on Trump heading into the weekend before Super Tuesday.

Rubio continued the verbal assault on Friday, repeatedly calling Trump "a con artist" in interviews designed to weaken Trump's tightening grasp on the Republican nomination. And a new advertising campaign from Rubio allies attacked Trump's business background and limited knowledge on foreign policy.

"A con artist is about to take over the Republican Party and the conservative movement," Rubio charged on CBS' "This Morning." ''And we have to put a stop to it."

But as he has so many times during the campaign, Trump found a way to turn the attention back to himself just hours later, with the Christie endorsement. "I just said wow, that is really great," Trump said of the governor's support.

Trump has won three consecutive primary contests and leads polls in most of the 11 states participating in Tuesday's mega-round of voting. Christie's endorsement is a blow to Trump's Republican competitors, not least Rubio, who had been courting the tough-talking New Jersey governor since he dropped his own presidential candidacy earlier in the month.

"Desperate people do desperate things," Christie said of Rubio's attacks while standing at Trump's side. "The idea that Marco Rubio can get inside Donald Trump's head is an interesting proposition."

Rubio's strategic shift comes as the GOP presidential candidates barreled into the final sprint to Super Tuesday. This, after a name-calling, insult-trading, finger-pointing debate Thursday night at which Rubio and Ted Cruz engaged in a tag-team attack against the GOP front-runner.

Trump's rivals concede that it might take an extraordinary "brokered" national Republican convention in July to stop him.

"Point is, there's still a pathway to beating him," said Liz Mair, a Republican strategist leading an anti-Trump group. "As of Wednesday, I wasn't convinced there really was."

On the Democratic side, Hillary Clinton claimed new headway on the eve of a South Carolina Democratic primary that she's expected to win handily.

"I think it does take me a little bit longer to get into the rhythm of campaigning," she said on MSNBC's "Morning Joe." ''We hit our stride in Nevada. Our message of breaking all barriers is really beginning to take hold. I really felt we were on an upward trajectory."

Even if the Republican-backed Trump attacks don't work in the short-term, Clinton suggested he'd be vulnerable as the GOP nominee in the fall.

"When it moves to the general election, I think you're going to see a real seriousness," she said, "with people turning and saying, 'What do we know about this man?'"

There was little sign that Trump's stock was falling at the Fort Worth rally where supporters gathered.

Many Trump supporters said they hadn't seen the Houston debate that shook him up. Those who had stood by their candidate and criticized the way his rivals teamed up on him.

"They come prepared to attack Trump at all costs," said Arlene Smart, 58, of Liberty, Texas. "I'm sick of lying in Washington. Trump is the man. That's all there is to it."

"I think most people see it as Washington's crooked attacks," she added.

As Trump's rivals stepped up their criticism, a pro-Rubio super PAC announced plans to start running Trump attack ads in key states on Friday.

One ad charges that Trump "knows nothing about foreign policy." Another targets his business background, highlights the businessman's use of "sleazy bankruptcy laws to avoid paying workers" and calls him out for saying he loves "the poorly educated" -- Trump's clumsy way of expressing compassion for the poor.

From Houston, the GOP candidates spread out in the hunt for Super Tuesday votes, with Cruz headed for Tennessee and Virginia on Friday. Both Trump and Rubio are signaling they're unwilling to cede Texas, the crown jewel of Tuesday's voting, to Cruz, the home-state senator.

Each scheduled campaign events in Texas before going to Oklahoma City.

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Peoples reported from Atlanta. Associated Press writers Nancy Benac and Laurie Kellman in Washington contributed to this report.