Democratic presidential front-runners Bernie Sanders and Pete Buttigieg came under sustained attacks during a debate as rivals raised questions about their ideology and experience.
The other contenders were hoping to sow doubts about whether Mr Sanders and Mr Buttigieg could defeat President Donald Trump.
Reeling from a weak finish in this week’s Iowa caucuses, former Vice President Joe Biden was a chief aggressor throughout Friday night’s debate.
He questioned Mr Sanders’s status as a democratic socialist and said Mr Buttigieg, the 38-year-old former mayor of South Bend, Indiana, does not have the background to lead in a complicated world.
Donald Trump lies all the timeBernie Sanders
Senator Amy Klobuchar of Minnesota, who is struggling to break into the top tier, echoed those criticisms.
But Mr Sanders and Mr Buttigieg, who essentially tied for first in the Iowa caucuses, largely brushed off the broadsides.
“Donald Trump lies all the time,” Mr Sanders said in response to suggestions that Mr Trump would use his self-described identity as a democratic socialist to brand him – and all Democrats – as radical.
Mr Buttigieg sought to portray himself as a fresh face from outside Washington with experience in dealing with real-life problems and ready to lead a weary nation in a new direction.
“I’m interested in the style of the politics we need to put forward to actually finally turn the page,” he said.
The debate was the eighth and perhaps most consequential in the Democratic Party’s year long quest for a presidential nominee.
I don’t know what about the past about Barack Obama and Joe Biden was so badJoe Biden
It came just four days after Iowa’s chaotic caucuses – and four days before New Hampshire’s primary – with several candidates facing pointed questions about their political survival.
Mr Biden was especially explicit about what was at stake for his candidacy during the opening moments, predicting he would “take a hit” in New Hampshire next week before the contest moves into more diverse states where he hopes to perform better.
He faced criticism on stage as someone too steeped in the ways of Washington to represent the change many Democratic voters say they are seeking. He responded by once again aligning himself with former President Barack Obama.
“The politics of the past I think were not all that bad,” Mr Biden said. “I don’t know what about the past about Barack Obama and Joe Biden was so bad.”
But Mr Biden had to defend his long record as the candidates sparred over the decision nearly two decades ago to send US troops to Iraq.
Mr Biden acknowledged that his vote in favour of the war authorisation as a senator was a mistake, while Mr Sanders said his Senate vote against deploying troops was proof of his judgment on national security issues.
Mr Buttigieg, who was in college at the time and later served in Afghanistan, said he would have opposed the war, too.
When a moderator asked Ms Klobuchar to respond to Hillary Clinton’s comments that no one likes Mr Sanders, Mr Biden walked over and gave him a hug.
Ms Klobuchar, meanwhile, joked that Mr Sanders is “just fine” and noted times when they had worked together on policy.
A sombre Mr Biden was appreciative when Mr Buttigieg defended him and his son, Hunter Biden, against attacks from Mr Trump in the impeachment inquiry.
Ms Warren avoided any direct criticism of her rivals and repeatedly pivoted to her core anti-corruption message.
As Mr Biden, Mr Sanders and Ms Klobuchar fought about the best way forward on health care, Ms Warren did not engage, instead speaking broadly about the need to lower prescription drug costs.
Billionaire activist Tom Steyer and New York entrepreneur Andrew Yang, meanwhile, were fighting to prove they belong in the conversation.
The stakes were particularly high for Mr Biden, who has played front-runner in virtually every one of the previous seven debates but left Iowa in a distant fourth place.
While reporting problems have blunted the impact of the Iowa contest, Mr Biden’s weakness rattled supporters who encouraged him to take an aggressive tack on Friday night.
The seven-person field highlighted the evolution of the Democrats’ 2020 nomination fight, which began with more than two dozen candidates and has been effectively whittled down to a handful of top-tier contenders.
There are clear dividing lines based on ideology, age and gender. But just one of the candidates on stage, Mr Yang, was an ethnic minority.