HOUSTON • Mr Joe Biden parried attack after attack from liberal rivals on everything from healthcare to immigration in a debate that showcased profound ideological divides between the Democratic Party's moderate and progressive wings.
The prime-time debate on Thursday night 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 - Mr Bernie Sanders and Ms Elizabeth Warren, who are his closest rivals - assailed him and his policies.
Unlike prior debates, where Mr Biden struggled for words and seemed surprised by criticism from his fellow Democrats, he largely delivered crisp, aggressive responses this time. He called Mr Sanders "a socialist", a label that could remind voters of the senator's embrace of democratic socialism. And Mr Biden slapped at Ms Warren's proposed wealth tax.
A two-term vice-president under Mr 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 healthcare reform.
His vulnerabilities surfaced, however, in the final minutes of the debate, when he was pressed on a decades-old statement regarding school integration.
The debate came as polls show a strong majority of voters believe the US is headed in the wrong direction under President Donald Trump. But nine months into their nomination fight, Democrats have yet to answer fundamental questions about who or what the party stands for beyond simply opposing Mr Trump.
The party's 2020 class, once featuring two dozen candidates, has essentially been cut in half by rules requiring higher polling and fund-raising standards. Just 10 candidates qualified for Thursday's debate, though more than that have qualified for next month's round.
Those in the second tier, after Mr Biden, Ms Warren and Mr Sanders, are under increasing pressure to break out of the pack. They all assailed Mr Trump. New Jersey Senator Cory Booker called Mr Trump a racist. Former Texas representative Beto O'Rourke called him a white supremacist. And California Senator Kamala Harris said Mr Trump's hateful social media messages provided "the ammunition" for recent mass shootings.
Mr Biden bears responsibility for millions of Americans going bankrupt under the "Obamacare" healthcare system, Mr Sanders also insisted. Former housing secretary Julian Castro raised questions about the Obama-Biden record on immigration, particularly the number of deportations that took place.
Mr Castro, a 44-year-old Texan, appeared to touch on concerns about Mr Biden's age when he accused the former vice-president of forgetting a detail about his own healthcare plan. At 76, Mr Biden would be the oldest president ever elected to a first term if he wins.
"Are you forgetting what you said two minutes ago?" Mr Castro asked. "I can't believe you said two minutes ago that you have to buy in and now you're forgetting that."
As well as policy differences, the Democratic debates have been shaped by broader questions about diversity. In a nod to the diverse coalition they need to defeat Mr Trump, the Democrats held this debate on the campus of historically black Texas Southern University. It unfolded in a rapidly changing state that Democrats hope to eventually bring into their column.
Some Democrats still fear that anyone other than a white man may struggle in a head-to-head match-up against Mr Trump. Mr Biden was one of four white men on stage.
Gun violence was another flashpoint in the debate. "Hell yes, we're gonna take your AR-15, your AK-47," Mr O'Rourke said, as the crowd cheered.
The national economy got surprisingly little attention, though several of the candidates criticised Mr Trump on foreign trade and his trade war with China.
Mr Trump was silent on social media during the event. Earlier in the day, he said he would likely have to watch a re-run.