HOUSTON (AP) — Joe Biden parried attack after attack from liberal rivals Thursday night on everything from health care to immigration in a debate that showcased profound ideological divides between the Democratic Party's moderate and progressive wings.

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.

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 Biden struggled for words and seemed surprised by criticism from fellow Democrats, he largely delivered crisp, aggressive responses. He called Sanders "a socialist," a label that could remind voters of the senator's embrace of democratic socialism. And Biden slapped at Elizabeth Warren's proposed wealth tax.

A two-term vice president under Barack Obama, 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," Biden declared.

His vulnerabilities surfaced, however, in the final minutes of the debate, when he was pressed on a decades-old statement regarding school integration. Biden rambled in talking about his support of teachers, the lack of resources for educators and at one point seemed to encourage parents to play records for their children to expand their vocabulary before segueing into talk of Latin America.

"That's quite a lot," quipped Julian Castro, the former Housing secretary who was Biden's frequent foe during the debate.

The candidates debated with polls showing a strong majority of voters believe the country is headed in the wrong direction under the first-term Republican 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 President Donald Trump.

The party's 2020 class, once featuring two dozen candidates, has essentially been cut in half by party rules requiring higher polling and fundraising standards. Just 10 candidates qualified for Thursday's affair, though more than that have qualified for next month's round.

Those in the second tier, after Biden, Warren and Sanders, are under increasing pressure to break out of the pack. They all assailed Trump.

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 vision among us, and that's why we've gotten nothing done," Harris charged.

In addition to Trump, Biden's rivals also turned against Obama's legacy at times as they sought to undermine the former vice president's experience.

Sanders insisted that Biden bears responsibility for millions of Americans going bankrupt under the "Obamacare" health care system. Castro raised questions about the Obama-Biden record on immigration, particularly the number of deportations that took place.

Castro, a 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?" an incredulous Castro asked, challenging Biden on health care. "I can't believe that you said two minutes ago that you have to buy in and now you're forgetting that."

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.

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 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.

The party cheered when America elected the most diverse congressional class in history in last fall's midterm voting. But some Democrats still fear that anyone other than a white man may struggle in a head-to-head matchup against Trump.

Biden was one of four white men onstage.

Along with health care, gun violence emerged as a flashpoint Thursday night in a state shaken by a mass shooting last month that left 22 people dead and two dozen more wounded.

O'Rourke noted 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.

The national economy got surprisingly little attention, though several of the candidates criticized Trump on foreign trade and his trade war with China.

South Bend, Indiana, Mayor Pete Buttigieg said Trump had said scornfully of his candidacy "he'd like to see me making a deal with Xi Jinping," the Chinese president.

"I'd like to see HIM making a deal with Xi Jinping."

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!"

Earlier in the day, Trump said he'd likely have to watch a re-run because of travel conflict. He predicted the Democratic nominee would ultimately be Biden, Warren or Sanders.

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Peoples reported from Washington. AP writer Colleen Long contributed to this report.