Joe Biden in the firing line as Democratic candidates debate on TV
Mr Biden defended his former boss Barack Obama.
Joe Biden parried attack after attack from liberal rivals on health care and immigration in a fiery Democratic debate.
The televised tussle showcased profound ideological divides between the Democratic Party’s moderate and progressive wings ahead of next year’s presidential election.
The prime-time debate also elevated several struggling candidates, giving them a chance to introduce themselves to millions of Americans who are just beginning to follow the race.
Mr Biden dominated significant parts of the evening, responding strongly when the liberal senators who are his closet rivals — Bernie Sanders and Elizabeth Warren — assailed him and his policies.
Unlike prior debates, where he struggled for words and seemed surprised by criticism from fellow Democrats, Mr Biden largely delivered crisp, aggressive responses.
He called Mr Sanders “a socialist,” a label that could remind voters of the senator’s embrace of democratic socialism.
A two-term vice president under Barack Obama, Mr Biden unequivocally defended his former boss, who came under criticism from some candidates for deporting immigrants and not going far enough on health care reform.
“I stand with Barack Obama all eight years, good bad and indifferent,” he declared.
The candidates debated with polls showing a strong majority of voters believe the country is headed in the wrong direction under president Trump’s leadership.
But nine months into their nomination fight, divided Democrats have yet to answer fundamental questions about who or what the party stands for beyond simply opposing the incumbent.
Hell yes, we're gonna take your AR-15, your AK-47 Beto O'Rourke
The party’s 2020 class, once featuring two dozen candidates, has essentially been cut in half by party rules requiring higher polling and fund-raising standards.
Along with health care, gun violence emerged as a flashpoint on Thursday night in Texas, a state shaken by a mass shooting last month that left 22 people dead and two dozen more wounded.
Mr O’Rourke noted that there were not enough ambulances at times to take all the wounded to the hospital.
“Hell yes, we’re gonna take your AR-15, your AK-47,” he said, as the crowd cheered.
Minnesota Senator Amy Klobuchar noted that all the candidates on stage favour a ban on assault weapons and high-capacity magazines.
The national economy got surprisingly little attention, though several of the candidates criticised president Trump on foreign trade and his trade war with China.
Mr Trump was silent on social media during the event.
But Kayleigh McEnany, his campaign’s national press secretary, said in a statement: “Thank you to ABC and the Democrat Party for another infomercial for President Trump!”