The Republican front-runner on Friday hit back at the Florida senator who rattled him in a CNN debate the night before by unveiling the bombshell endorsement of New Jersey Gov. Chris Christie on an extraordinary day when the already wild 2016 presidential campaign turned into a political bloodbath.
Christie showed up unannounced here to laud Trump and savage Rubio, drawing gasps from reporters, after concluding the billionaire was destined to win the Republican nomination and was the best person to slug it out with Hillary Clinton for the White House.
"I will lend my support between now and November in any way for Donald," Christie told shocked reporters before Trump fired up a rally in the state with most delegates up for grabs in the Super Tuesday primaries next week.
"Generally speaking, I'm not big on endorsements," Trump said, adding, "This was an endorsement that really meant a lot."
In the short term, Christie's move put a halt to Rubio's victory lap after his stunning prosecution of Trump's character, background and business career in Thursday night's GOP debate, the most effective takedown of the billionaire so far in this campaign.
In the longer term, Christie's support equips Trump with a powerful and outspoken surrogate who can match the former reality star's tart tongue and, like him, can inflame a media narrative critical of his opponents.
Trump accompanied his rollout of Christie with a stream of invective against Rubio, reflecting the raised stakes ahead of Super Tuesday and the scorched-earth duel now raging between them.
As the campaign headed further toward the gutter, Trump branded Rubio a "low life," "a nasty little guy," a "basket case" and "a choker" who sweated so much he had to put makeup on with a trowel.
The Florida senator, meanwhile, punched back, seeming to relish his new aggressive campaign mode. He suggested Friday morning that Trump was a "con artist" who was worried about wetting his pants and mocked the serial tweeter's spelling before a delighted crowd in Texas.
A source close to Christie said the governor's decision to jump back into the campaign alongside Trump just two weeks after bowing out after failing to feature in New Hampshire was the result of a number of factors.
Christie has now concluded that Trump will indeed become the GOP's nominee after three consecutive wins, leaving him well positioned going into Super Tuesday. There is also little to suggest, in Christie's mind, that any other candidate can stop Trump.
Therefore, the source said, Christie now believes it's time for the Republican Party to unite around its likely nominee for what figures to be a bitter battle with Clinton, the most likely Democratic nominee.
Furthermore, the more moderate and pragmatic Christie has never been on the same ideological wavelength as Texas Sen. Ted Cruz, and the disdain he clearly feels for Rubio is genuine and deep, the source said.
Although his own campaign faltered, and his belief that the next president should be a former governor with executive experience looks unlikely to be realized, Christie still believes that an executive -- like Trump -- would be the best fit for the White House.
Another source said that Christie had examined the state of the race and believed Ohio Gov. John Kasich and former neurosurgeon Ben Carson had no viable path forward.
Yet, despite dropping out of the race, the source said the New Jersey governor wanted to be a factor in the campaign, and after his evisceration of Rubio in a debate in New Hampshire, he relishes the role of being Trump's top attack dog.
It may be no coincidence that a vice presidential nominee takes on that role. There will now be feverish speculation that Christie could end up alongside Trump on the GOP ticket.
But Christie would also be a possible fit for other jobs in a Trump cabinet, including attorney general or secretary of Homeland Security. Christie, as a more experienced political player than Trump, may also feel that he is now in a position to influence the direction and tone of the billionaire's often raucous campaign.
There is also a personal dimension to the endorsement. Christie and his wife Mary Pat have been friendly with the Trump family for more than a decade.
But the governor's move did cause some consternation in his inner circle, the source said, saying that some of his donors and campaign operatives were shocked. One such person said in a text message that he was "stunned" at his former boss's move.
Christie's decision came at a time when establishment figures and donors are flocking to Rubio in the apparent belief that he is the only Republican who can stop Trump, who many leaders believe could be a disaster in a general election.
It is not the first time that Christie's maneuvering has caused uproar in a general election. His cozying up to President Barack Obama in the 2012 election after Hurricane Sandy infuriated GOP nominee Mitt Romney's campaign.
Kasich, Rubio teams respond
A source close to Rubio's campaign dismissed Christie's endorsement.
"Who cares? (Christie) has zero support. Zero finance types," the source said.
Another Rubio source said, "Christie is the invisible man to Republican primary voters."
And senior Rubio aide Todd Harris poked fun at Trump for the endorsement.
"Gov. Christie is a very articulate guy which is probably why Donald Trump had to bring him on to his campaign," Harris said. "Clearly Donald Trump is incapable of explaining any of his ideas for the future of this country, he's incapable of explaining what's hiding in his tax return."
A staffer for Kasich explained the Christie endorsement this way: "Marco hate."
Another Kasich staffer said Christie, who is friends with Kasich and received his early backing to lead the Republican Governors Association, didn't call the Ohio governor ahead of Friday's endorsement.
"It doesn't change anything for us," Kasich campaign spokesman Chris Schrimpf said. "We are still the only adult in the race who is capable of running the country. Seems like a nightmare for Rubio, though."
When asked about the endorsement, Cruz dodged on weighing in.
"This decision is going to be made by the voters on Super Tuesday," Cruz told Fox News' Sean Hannity in Nashville, Tennessee.
'That part is over'
While Christie profusely lauded Trump during the news conference Friday, the New Jersey governor known for his similarly brash style had previously slammed Trump as an "entertainer in chief" during the Republican primary race and called Trump's plan to temporarily ban Muslims from the United States "ridiculous."
"That part is over," Christie said of the fighting between the two men during the primary race, adding that his decision to endorse Trump came down to "who is the best person to stop Hillary Clinton from getting inside the White House ever again."
He added: "There is no one who is better prepared to provide America with the strong leadership that it needs."
Trump and Christie appeared on stage together the day after a tense debate in which the real estate mogul faced off against Rubio and Cruz. Christie and Trump took turns unloading on the freshman Florida senator, who spent the morning insulting the billionaire businessman.
"Desperate people do desperate things," Christie said when asked about Rubio's comments, adding that his remarks were the "flailing punches in the final days of the campaign."
Christie, in the waning days of his own presidential campaign, had specifically targeted Rubio as not qualified for the office. And he kept up his attacks when he introduced Trump at a campaign rally in Oklahoma City later Friday.
"Marco Rubio, your campaign is almost over, buddy," Christie said.
Christie said he decided Thursday to join Trump on the campaign trail.
"Yesterday morning I met with Donald and we sat and talked and he said, 'How about coming out to Texas with me?' and I said, 'Happy to do it, whenever you're ready sir,'" Christie said.
A Christie adviser said the two met Thursday at Trump Tower before the debate and Christie made up his mind to endorse either on the spot or soon after. Christie only told a small group of people Thursday night, but not until he was boarding a flight to Texas.
Asked about whether he would consider Christie as his running mate should he win the nomination, Trump said he didn't "want to discuss that," but added: "He's certainly got the talent."
Trump has previously said that he would pick a running mate with political experience, and not someone from the private sector like himself.