"This is America," said Biden, his party's early front-runner, before calling Sanders "a socialist." He later declared, "I stand with Barack Obama all eight years, good bad and indifferent."
The top White House hopefuls faced off for the first time alongside seven other candidates who are under increasing pressure to break out of the pack. All assailed Trump without mercy.
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New Jersey Sen. Cory Booker called Trump a racist. Former Texas Rep. Beto O'Rourke called him a white supremacist. And Kamala Harris, a California senator, said Trump's hateful social media messages provided "the ammunition" for recent mass shootings.
"President Trump, you have spent the last two-and-a-half years full time trying to sow hate and division among us, and that's why we've gotten nothing done," Harris declared.
Get the latest updates and livestream video on the third Democratic Debate live from ABC News
The Democrats primary contest has been remarkably stable, but the debate comes at a pivotal point in the campaign as more voters move past their summer vacations and begin to pay closer attention to politics.
Polls show that a strong majority of all voters believe the country is headed in the wrong direction under the first-term president'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 Trump.
The debate is shaped by evolving issues of race, gender, generation and ideology that again exploded into public view on the debate stage Thursday night.
Obama himself emerged as a hot point as the discussion shifted to health care and immigration in particular.
Sanders said Biden, Obama's two-term vice president, bears responsibility for millions of Americans going bankrupt under the "Obamacare" health care system.
Biden slapped back at both Sanders and Warren and contended they haven't yet explained how they would pay for Sanders' government-backed "Medicare for All" health care plan.
Bernie Sanders on his clashes with Joe Biden: "Joe is a good friend of mine. I like him very much. He's a very decent human being. But Joe and I have a very different voting record—and a very different vision for this country." https://t.co/INdRXlIwFs #DemDebate pic.twitter.com/IxpUYnJH2d— ABC News (@ABC) September 13, 2019
Castro, who served as Obama's housing chief, kept the pressure on the front-runner.
The 44-year-old Texan appeared to touch on concerns about Biden's age when he accused the former vice president of forgetting a detail about his own health care plan. At 76, Biden would be the oldest president ever elected to a first term.
"Are you forgetting what you said two minutes ago?" Castro asked. "I can't believe that you said two minutes ago that you have to buy in and now you're forgetting that."
The first question of the third #DemDebate was on health care.— ABC News (@ABC) September 13, 2019
"I think we should have a debate on health care. I know that the senator says she's for Bernie, well I'm for Barack, I think the Obamacare worked," former VP Joe Biden said. https://t.co/INdRXm0842 pic.twitter.com/TsvwApU6Dy
He added: "I'm fulfilling the legacy of Barack Obama and you're not."
The ABC News debate was the first limited to one night after several candidates dropped out and others failed to meet new qualification standards. A handful more candidates qualified for next month's debate, which will again be divided over two nights.
Mayor Pete Buttigieg on education: "If we want to get the results that we expect for our children, we have to support and compensate the teaching profession. Respect teachers the way we do soldiers and pay them more like the way we do doctors." https://t.co/INdRXm0842 #DemDebate pic.twitter.com/Gb0M9eHwlh— ABC News (@ABC) September 13, 2019
Besides the infighting, viewers saw the diversity of the modern Democratic Party.
The debate, held on the campus of historically black Texas Southern University, includes women, people of color and a gay man, a striking contrast to the Republicans. It unfolded in a rapidly changing state that Democrats hope to eventually bring into their column.
The debate shifted to gun violence in a state shaken by a mass shooting last month that left 22 people dead and two dozen more wounded.
"It seemed to me that Biden was right and that Julián Castro mischaracterized what he had said," @jonkarl says on Castro's health care quip to Biden that the former vice president was "forgetting" what he said "two minutes ago" https://t.co/T37EaVOvlU #DemDebate pic.twitter.com/MHhey9PShr— ABC News Politics (@ABCPolitics) September 13, 2019
In an emotional moment, O'Rourke said that there weren't 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 Sen. Amy Klobuchar noted that all the candidates on stage favor a ban on assault weapons and high-capacity magazines. She favors a voluntary buy-back program on assault weapons, however.
Meanwhile, Trump noted he'd be in Baltimore at a Republican retreat during the debate and wasn't sure he'd get a chance to watch. But he predicted the Democratic nominee would ultimately be Biden, Warren or Sanders.
"It's going to be very interesting," Trump said. "I'm going to have to watch it as a re-run."
The leading Democratic candidates may not have another chance to face off for quite some time.
The Democratic field may be divided into two groups when they meet in October because more than 10 candidates qualified for the next round.