Surging Republican hope Marco Rubio wilted under stained attack in the last US presidential debate before the New Hampshire primary as frontrunner Donald Trump insisted he could be commander in chief.
The telegenic, 44-year-old Florida senator, who polls suggest would have the best chance of winning the White House for the Republicans, was savaged by his governor rivals for his lack of experience, appearing to flounder on a debate stage where he often shines.
If Rubio puts in a strong showing however at the polls, capitalising on his strong third-place finish in Iowa, it will likely confirm him as the establishment candidate of choice for the nomination.
He has announced a string of new congressional endorsements all week, including former governors of Louisiana and New Hampshire on Saturday, projecting a sense of confidence and mainstream electability.
The most sustained attack was waged by New Jersey Governor Chris Christie, whose presidential dreams depend on outperforming him on Tuesday and who has denigrated Rubio for being controlled by his team.
"I like Marco Rubio and he's a smart person and a good guy, but he simply does not have the experience to be president of the United States and make these decisions," said Christie.
"Marco, the thing is this. When you're president of the United States... the memorised 30-second speech where you talk about how great America is at the end of it doesn't solve one problem."
Rubio was booed for lashing back by accusing Christie of dragging his feet over leaving the campaign trail when his state was hit by a deadly East Coast snowstorm last month.
"You didn't even want to go back. They had to shake you into going back. You stayed there for 36 hours and then he left and came back," Rubio said.
Christie returned fire laden with sarcasm: "It gets very unruly when he gets off his talking points."
Jeb Bush, whose dreams of following his father and brother into the White House will likely live or die on Tuesday, sharpened his performance, questioning Rubio's experience and butting heads with Trump.
"Leadership. You learn this, you learn it by doing it. It's not something that you just go up, and on the job do it," said Bush.
In New Hampshire, Trump commands 35 percent of support among likely Republican voters, a 21-point lead over Rubio on 14 percent, according to the latest 7News/University of Massachusetts Lowell poll.
Texas Senator Ted Cruz, the evangelical conservative who won the Iowa caucuses was on 13 percent, but with nine percent undecided, there is everything to play for.
New Hampshire's famously independently-minded voters are proud of the role they play in the first primary that gives the winners enormous momentum going forward and can mortally puncture losers' ambitions.
Even if Trump does win the Republican primary, the eyes of the party establishment, appalled by his insults, incendiary rhetoric and lack of political experience, will likely focus on his moderate rivals.
Bush, Christie and Ohio Governor John Kasich must make a mark in New Hampshire and deflate the growing bubble around Rubio if they still have a chance to capture the support of establishment Republicans.
Bush, whose mother has fondly chastised him for being too polite, laid into Trump for allegedly trying to take the property of an elderly woman in Atlantic City.
"That is downright wrong," said Bush.
"Jeb wants to be a tough guy tonight," shot back Trump, who is chasing a win in the primary after finishing second in the Iowa caucus this week.
"How tough it is to take property from an elderly woman?" replied Bush.
"Let me talk. Quiet," replied Trump to boos.
Kasich urged voters to pick him.
"If I get elected president, head out tomorrow and buy a seat belt, because there's going to be so much happening in the first 100 days, it's going to make your head spin," he said.
Trump, who boycotted the previous debate, stood center stage determined to cement his lead in the local polls even if a Quinnipiac University poll says 30 percent of the party would not support him.
"I actually think I have the best temperament," said Trump, kicking off the debate when asked about criticism from Cruz that he would be too eager to press the nuclear button if commander in chief.
The real estate tycoon has whipped up a passionate following among white blue collar Americans, fed up with career politicians and increasingly frustrated by struggling to make ends meet.
"We have galvanised and created a movement," said Trump when asked how he would counter Democrat frontrunner Hillary Clinton's quest to make history by becoming the first woman president of the United States.