Seven Republicans seeking the party's nomination to run for the White House are having a fractious
TV debate in New Hampshire, days before voters there make their presidential picks.
New Jersey Governor Chris Christie tore into Senator Marco Rubio as inexperienced and over-scripted.
Mr Rubio came third in Iowa and is seen as a rising force.
Ted Cruz is looking to build on his victory in the Iowa caucuses ahead of Tuesday's New Hampshire vote.
It will be the second in the state-by-state contests to pick a presidential nominee in the Republican and Democratic parties.
One of the testiest moments 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 but Mr Rubio attacked him for not immediately returning to New Jersey from the campaign trail during the recent snowstorm.
Debate highlights so far:
New York billionaire Donald Trump, who leads the New Hampshire polls, said he would bring back "worse than waterboarding"
Mr Rubio accuses President Obama of changing America and making it "like the rest of the world", prompting a savaging from Mr Christie for trotting out "rehearsed soundbites"
Ohio Governor John Kasich says Mr Trump's plan to deport 11m undocumented immigrants would break up families and it's "not the America I know"
Socialised medicine in other countries doesn't work and leads to rationing, says Mr Cruz, in an attack on Mr Obama's healthcare reform
Mr Trump is booed for telling Jeb Bush to be quiet during an angry exchange over government seizure of private property
Mr Cruz apologised to Ben Carson after staff told voters in Iowa that the retired neurosurgeon was going to end his campaign
Ben Carson missed his entrance to the debate and was left standing in the wings, in a very awkward opening
The results of Tuesday's New Hampshire primary are expected to determine whether or not some of the Republican candidates drop out.
The Iowa caucuses narrowed the field, with Kentucky Senator Rand Paul and former Pennsylvania Senator Rick Santorum withdrawing from the race after the results were announced.
ABC News, which is hosting the debate in Manchester, New Hampshire, has dropped an undercard event for low-polling candidates, leaving former Hewlett-Packard CEO Carly Fiorina as the only remaining candidate without a spot on stage.
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.