Ted Cruz and Marco Rubio, first-term senators on the rise in the US presidential race, faced a barrage of attacks in Saturday night's Republican TV debate ahead of the New Hampshire primary.
Rubio exceeded expectations to finish third in the Iowa caucuses and appears to be gaining steam heading into Tuesday's primary.
His rise is a threat not only to frontrunners Donald Trump and Cruz but to several other candidates, including Jeb Bush, who need a strong showing in New Hampshire to stay in the campaign.
New Jersey Governor Chris Christie took immediate aim at Rubio on Saturday night, saying that the Florida senator has "not been involved in a consequential decision where you need to be held accountable".
Bush, in turn, said Rubio was a gifted politician but warned voters against again putting the White House in the hands of a first-term senator: "We've tried it the old way, with Barack Obama and soaring rhetoric," he said.
Al Jazeera's Alan Fisher, reporting from the debate hall in New Hampshire, said that "Christie attacked [Rubio] quite early on in the debate, and he never really seemed to recover". 
Rubio said he was proud of his service in the Senate and suggested that Obama's "problems" were less about experience and more about ideology.
He also defended his decision to walk away from the sweeping immigration bill he originally backed in the Senate and said he would not pursue similar legislation as president.
"We can't get that legislation passed," Rubio said of the bill that would have provided a pathway to citizenship for millions of people in the US illegally.
'Washington ethics'
Cruz, who was the winner in Iowa, also faced criticism for messages his campaign sent to voters ahead of the caucuses, saying rival Ben Carson was dropping out and urging the retired neurosurgeon's supporters to back Texas senator instead.
Cruz apologised for his campaign's actions on Saturday, but not before Carson jabbed him for having "Washington ethics". Those ethics, he said, "say if it's legal, you do what you do to win".
Trump was back on the debate stage after skipping the last contest before the Iowa caucuses.
After finishing second in Iowa, he sought to refocus on the core messages of his campaign, including blocking Muslims from going to the US and deporting all people in the country illegally.
Trump currently leads the polling in New Hampshire, but the debates have heavily shifted support for candidates in the past, Fisher reported.
"Marco Rubio will spend the next 72 hours scanning the poll numbers to make sure no lasting damage was done at the debate here," Fisher added.
The debate began shortly after North Korea defied international warnings and launched a long-range rocket that the UN and others call a cover for ballistic missile test.
Asked how he would respond to North Korea's "provocations", Bush said he would authorise a pre-emptive strike against such rockets if it was "necessary to keep America safe".
Cruz said he would not speculate about how he would handle the situation without a full briefing, whilst Trump said he would rely on China to "quickly and surgically" handle North Korea.