Marco Rubio and Donald Trump emerged Friday as the principal antagonists in an all-out brawl for the future of the Republican Party, as establishment opposition to the front-runner's candidacy started to crumble with a high-profile endorsement by New Jersey Gov. Chris Christie.

During one of the most hectic and consequential days of the campaign, Rubio dramatically escalated his verbal assault of the real estate mogul, branding him a "con artist" attempting to hijack the party, and the senator from Florida attracted a wave of endorsements. Trump responded with name-calling of his own and by touting the support of Christie, who dropped out of the presidential race after a poor showing in New Hampshire.

The row brought into focus the identity crisis gripping the GOP, pitting those eager to embrace the bravado and nativist message of Trump against those who view him as an existential threat who will drive away minorities and moderates. The endorsement by Christie, a former head of the Republican Governors Association, could prompt other establishment Republicans to fall behind the front-runner.

"This is a real signal to the establishment that they better start thinking in a positive way about how they are going to work with candidate Trump and President Trump," said former House speaker Newt Gingrich. Gingrich, who has not endorsed anyone, said Friday marked the "beginning of the end" for anti-Trump forces.

The rapid escalation in hostility between Rubio and Trump, who had mostly avoided each other for months, sets the stage for a brutal 2 1/2 weeks that could determine whether Trump will effectively clinch the nomination by mid-March or the race will drag on, perhaps all the way to the GOP convention in July.

A third presidential hopeful, Sen. Ted Cruz of Texas, embarked on an ambitious campaign swing Friday but was reduced to being a spectator in his home state as Rubio and Trump dueled about 30 miles apart from one another. At the 10th GOP debate in Houston the night before, Cruz was similarly overshadowed as Rubio began his concerted attacks on Trump.

Continuing the barrage Friday, Rubio unleashed a string of personal insults that seemed unthinkable when he launched his optimism-tinged campaign in the spring.

In essence, Rubio is trying to beat Trump at his own game, at a time when the senator has yet to win a primary contest and faces an extremely difficult path in a series of states seen as friendly to Trump. Rubio spent much of the debate and much of Friday hurling pre-packaged digs at Trump's appearance, intellect and trustworthiness as a businessman.

At a morning rally in Dallas, Rubio pulled out his smartphone and ridiculed Trump for misspelled words in his Twitter feed. He suggested there were only two explanations for the mess-ups.

"Number one: That's how they spell those words at the Wharton School of Business, where he went," Rubio said. "Or number two, just like Trump Tower, he must have hired a foreign worker to do his own tweets." (The misspelled tweets were later deleted and replaced with corrected versions.)

Rubio, 44, also highlighted the age gap between him and Trump, 69, saying "you start to worry" about whether the front-runner could serve as president for eight years.

He also accused of Trump requesting a full-length mirror during a break in the debate, "maybe to make sure his pants weren't wet." And he questioned Trump's toughness by charging that he was "the first guy that begged for Secret Service protection."

A few hours later in Fort Worth, Trump appeared with Christie at a news conference and a rally. Christie said there was "no one who is better prepared to provide America with the strong leadership that it needs" than Trump.

At the rally, Trump launched a series of character attacks against Rubio, calling him a "low-life," a "nervous basket case" and "a nervous nellie."

The billionaire also mocked Rubio's physical appearance, lampooning his backstage preparations for Thursday's debate. "He was putting on makeup with a trowel. I will not say that he was trying to cover up his ears. I will not say that," Trump said. "He was just trying to cover up - he was just trying to cover up the sweat."

Trump also issued another threat against the media Friday, vowing to "open up" federal libel laws to make it easier to sue news outlets such as The Washington Post and the New York Times.

While they are now on the same team, the intense primary campaign in the past year frayed the Christie-Trump relationship, with both candidates taking shots at each other. Christie frequently said Trump was too inexperienced, called Trump's proposal to ban Muslim immigrants "ridiculous" and said Trump's plan for a border wall "makes no sense." Trump responded by accusing Christie of being complicit in a bridge-closing incident in New Jersey, which Christie has denied.

But Christie showed particular animosity toward Rubio. In the last debate before he bowed out, Christie nearly derailed Rubio's presidential ambitions by mocking the flustered senator for repeating canned lines several times. Rubio came in fifth in the New Hampshire primary three days later.

Shortly after Christie got out of the race, Trump called the governor and aggressively pursued his endorsement, according to a longtime Christie adviser, who, like others, spoke on the condition of anonymity to describe private discussions.

According to people on both sides familiar with the talks, Christie and his wife, Mary Pat, made a quiet visit Thursday to Trump Tower in New York, where they met privately with Trump and his wife, Melania, for about an hour, talking through the race and Christie's endorsement.

After the meeting, Christie alerted a few people in his orbit of his decision but otherwise kept mum, since he and Trump wanted to preserve an element of surprise.

Christie confidant William Palatucci followed up his announcement Friday by holding a conference call with New Jersey's Republican leadership, including a number of county GOP chairmen and other power brokers, according to two people familiar with the call. Palatucci urged them to follow him and endorse Trump, calling the businessman the presumptive nominee.

Trump also received an endorsement Friday from Maine Gov. Paul LePage (R).

Rubio spent months mostly sidestepping Trump, as his strategists wagered that the best possible time to strike him was when the field had narrowed to a one-on-one race. As recently as Monday, Rubio explained to reporters on his campaign plane why he didn't tend to call out Trump by name at his events: "That's how I speak, and that's how my campaign's going continue to be."

But it was becoming increasingly clear the campaign didn't have the luxury of waiting for the field to slim down. Trump is on a three-state winning streak and leads in the polls in a slate of "Super Tuesday" states holding nominating contests on March 1. Rubio has yet to win a state, and his aides are not predicting victory anywhere on Tuesday.

"It was timing," a Rubio adviser said. "When Christie and [Jeb] Bush - the other establishment-friendly candidates - are still in the race . . . you're playing 12-way chess. You know that Candidate A attacks Candidate B and Candidate C benefits."

One of the most memorable moments of Thursday's debate came when Rubio said that if Trump had not received a generous inheritance, he would be "selling watches in Manhattan." It was Rubio who first brought the line up during debate preparations, according to a Rubio adviser, and those in the room laughed and encouraged him to use it.

Rubio repeated most of his attacks on Trump at a crowded event in Oklahoma City on Friday afternoon. The audience of about 1,000 joined in on the jokes. "He's so cheesy!" yelled one supporter after Rubio read a few of Trump's syntax-tangled tweets. But toward the back of the room, some voters who had fallen for Rubio long ago wondered if he would regret trading insults with the mogul.

Also on Friday, a super PAC supporting Rubio, Conservative Solutions PAC, released TV ads attacking Trump that will air in March primary states. And a separate nonprofit group, American Future Fund, released ads featuring people who said they were "scammed" and "hurt" by Trump University, which Rubio criticized at the debate.

Trump has collected by far the most delegates after decisive wins in New Hampshire, South Carolina and Nevada. He is the front-runner in most March 1 states, leading some detractors to fear that opponents waited too long to wage a full-fledged strike against him.

The one state where Rubio is predicting victory is delegate-rich Florida, which votes March 15 and awards its delegates on a winner-take-all basis. But polls show Trump leading Rubio in his home state by double digits.

"Why would the people of Florida vote for Marco Rubio when he defrauded them by agreeing to represent them as their Senator and then quit!" Trump tweeted Friday. Rubio has taken heat for missing many Senate votes. A 2015 study by Vocativ and Govtrack found Rubio to be the most often absent member of the Senate.

Rubio and his campaign have said that a contested convention is a possibility for which they are preparing. If no one wins a majority of delegates after all the states have held contests, the nomination fight would culminate in Cleveland in July.

The three top GOP contenders planned to spend the weekend focusing on "Super Tuesday" states. Cruz spent Friday in Nashville and Virginia Beach and will spend Tuesday night in Texas, where he desperately needs to win after three consecutive third-place finishes.

Rep. Peter King, R-New York, a combative Long Island moderate who has endorsed Rubio, said Friday that he was ready to hit the campaign trail to offer an alternative to the combined pugnacity of Trump and Christie, previewing of the newer, more bitter stage of the race that has just arrived.

"Where Donald Trump grew up, that's not the real Queens," King said. "He's not a tough guy from an outer borough. I grew up 20 minutes away in another world, and I know there's never been a real tough guy out of Jamaica Estates."