US election 2016: Rubio assailed by Republican rivals
Senator Marco Rubio was attacked by several Republican rivals for the party's nomination to run for the White House, in a fractious TV debate.
New Jersey Governor Chris Christie tore into Mr Rubio, a rising force in the polls, as inexperienced and scripted.
The Florida senator, also assailed by Donald Trump and Jeb Bush, fought back by attacking President Barack Obama.
The debate in New Hampshire comes four days before the state picks its presidential nominee in each party.
Several of the seven Republicans on stage have staked much on this state, and the pressure seemed to provoke several points of conflict.
One of the testiest moments at the debate in Manchester was when Mr Christie lambasted Mr Rubio.
"You have not been involved in a consequential decision, where you had to be held accountable, you just simply haven't," said Mr Christie.
He accused the Florida senator of "truancy" by missing Senate votes and said his eloquent speeches were ineffective in improving the lives of a single American.
But Mr Rubio, who was a strong third in the Iowa vote on Monday, attacked him for not immediately returning to New Jersey from the campaign trail during the recent snowstorm.
The night got off to a bizarre start when Ben Carson missed his entrance to the debate and was left standing in the wings by debate hosts ABC News.
When it eventually got under way, the candidates sharpened their differences on issues like national security, immigration and abortion.
- Donald Trump, who leads the New Hampshire polls, said he would bring back interrogation techniques "worse than waterboarding"
- He was booed for telling Jeb Bush to be quiet during an angry exchange over government seizure of private property
- Jeb Bush said he would take a pre-emptive strike against North Korea if necessary, following their rocket launch
- And he and Mr Christie broke ranks with the others on abortion, saying they backed it in cases of rape and incest
- Ohio Governor John Kasich slammed Mr Trump's plan to deport 11m undocumented immigrants, saying it would break up families
- Socialised medicine in other countries doesn't work and leads to rationing, said Ted Cruz, attacking Mr Obama's healthcare reform
- Mr Cruz apologised to Ben Carson for telling voters in Iowa that the retired neurosurgeon was going to end his campaign
Analysis - Anthony Zurcher, BBC News, New Hampshire
Marco Rubio was expecting a fight when he came out on to the debate stage tonight, and he got one.
New Jersey Governor Chris Christie launched into a full-scale assault on the Florida senator from the get-go, repeatedly accusing him of being too programmed and light on experience.
Mr Rubio may have thought he was prepared, but his responses played right into Mr Christie's hands. When Mr Rubio launched into one of his smooth, paragraph-long answers, the New Jersey governor was ready with a devastating blow.
"The memorised 25-second speech," Mr Christie observed. "There it is, everybody."
One of the senator's strengths in past debates has been his ability to relentlessly pivot his answers back to his campaign talking points, but Mr Christie had turned that skill into a weakness.
It's just a few days until the New Hampshire primary, and Mr Rubio was poised to consolidate mainstream Republican support with a strong showing. Mr Christie's goal was to make those voters think twice before marking their ballots - and he may have just done so.
New Hampshire polls suggest businessman Mr Trump, who has no political experience but finished second in the Iowa vote on Monday, is out in front.
Texas Senator Ted Cruz, who like Mr Trump is running on an anti-establishment platform, was the victor in Iowa.
But a strong third by Mr Rubio has made him a target from the others.
One of his counter-attacks under pressure in the debate, that President Obama is knowingly changing the nature of the US, was strongly rejected by Mr Trump.
"I think he [Mr Obama] has no idea what he's doing. And our country is going to hell. So, I just want to say, we disagree on that."
Mr Bush also put the boot into his former Florida protege, saying he was gifted but in a way that reminded him of the current president.
After New Hampshire on Tuesday, the rest of the 50 states will have their turn over the coming weeks and months.
Each party formally announces their presidential candidate at conventions in July, four months before the presidential election.
More on Marco Rubio and the White House race
- Rubio rising On the trail of rising Republican Marco Rubio
- Clinton's kryptonite? Why the Florida senator is the man Hillary might fear the most
- Why are Americans so angry? The underlying forces that explain rise of Trump, Cruz and Sanders
- How does a US election work? If you want to be president, it helps to be governor, senator, or five-star military general - and have lots of patience
- Special report: The BBC's full coverage of the race to the White House