By : Anthony Zurcher The warm-up rounds are over. For the first time, all the top Democratic presidential candidates were on the debate stage together and the results were crackling. No more theoretical discussions of how candidate x on day one looked compared to candidate y on day two. No more sparring with lower-tier candidates who served as surrogates for stronger candidates not in the room. No more Queensberry rules of boxing. It was a bare knuckle affair. Here are some of the biggest moments. Biden under attackThe first question, as it was during the earlier debates, was healthcare - an issue polls regularly show ranks high for American voters. The fault line within the Democratic Party currently sits between candidates who want to do away with private insurance in place of a nationalised health system and those who want a "public option" that competes with private insurers. Democratic frontrunners spar over healthcare That source of friction was on full display Thursday night. Joe Biden, sandwiched between the progressive duo of Elizabeth Warren and Bernie Sanders, sparred with them both. Regarding Sanders's assertion that businesses will give savings back to workers from not having to pay their insurance, the former vice-president quipped: "For a socialist you've got a lot more confidence in corporate America than I do." He wondered aloud how any of the progressive candidates would pay for their sweeping plans. Between Sanders and Warren, the latter offered the most skilful response. Sidestepping questions about whether her plan would raise middle-class taxes, she said the issue was the costs of the current system - in insurance premiums, out-of-pocket deductibles and medical care forgone because of the expense. "What this is about is making sure we have the most efficient way possible to pay for healthcare in this country," she said. The real fireworks erupted later, when the lower-tier candidates chipped in. Amy Klobuchar said Sanders' plan was a "bad idea". Pete Buttigieg said it "doesn't trust the American people" to choose between public or private options. Then Julian Castro lashed out at Biden, in perhaps the most jarring exchange of the night. The topic, whether poor Americans are automatically covered by government insurance, was beside the point. The critical moment came when Biden disagreed with Castro's assertion, and the former Obama cabinet secretary accused the former Obama vice-president of "forgetting already what you said just two minutes ago". For the first time a candidate all-but-directly questioned the 76-year-old Biden's mental faculties. Whether or not that jab leaves a permanent mark - and whether, if it does, it helps Castro - the glass has been broken. For the first time in a Democratic debate this year, gun control received an early and thorough discussion. The reason was tragedy - two mass shootings in Texas and one in Ohio. For the first time in years, however, Democrats were talking about aggressive new gun-control proposals - and none were doing so more vocally than Beto O'Rourke. The former congressman from El Paso, the location of one of those mass shootings, has called for expanded background checks for gun purchases and no restrictions on assault weapons - something all the candidates on the stage agree with. He's also advocated mandatory buybacks of the banned weapons - something he's the only one pushing. And he was very blunt. "Hell yes, we are going to take your AR-15, your AK-47," he said to applause from the debate hall. In the past, that sort of sentiment was seen by many Democrats as political poison. It still is controversial. And yet the topic of gun control has given O'Rourke's campaign an energy and vigour that was lacking as he's had his slow descent in the polls since he launched his campaign with much fanfare earlier this year. "A racism and violence that had long been a part of America was welcomed out into the open and directed to my hometown of El Paso, Texas, where 22 people were killed, dozens more grievously injured by a man carrying a weapon he should never have been able to buy in the first place, inspired to kill by our president," O'Rourke said in his opening statement. There's no guarantee O'Rourke can change the trajectory of his campaign at this point. But past debates have not been kind to the Texan. At least for one night, he stood out in a way that might help him.