Second-tier candidates Beto O'Rourke, Amy Klobuchar and Cory Booker enjoyed strong performances, but a highly-anticipated clash between frontrunners Joe Biden and Elizabeth Warren failed to materialize, as the top 10 Democrat candidates for the U.S. presidency clashed in Houston on Thursday night in the party's third debate of the primary season.

Here are five takeaways:

1. No Biden-Warren clash

Going into the debate, Elizabeth Warren, the senator from Massachusetts, had moved into second place in national polls behind frontrunner and former vice-president Joe Biden. A clash between the progressive populist and the moderate was highly anticipated, but did not materialize.

Warren had a steady but lower-key night and Biden was again the punching bag, taking blows from Vermont Senator Bernie Sanders and Julien Castro, housing and urban development secretary in the Obama administration, in particular.

Alluding to Biden's missteps on the campaign trail, Castro, struggling in the polls, asked "are you forgetting what you said two minutes ago?" after Biden appeared to flip-flop on an answer on healthcare. The critical response online suggested the move backfired on Castro.

The former vice president again mangled his words as the debate went on and made dated references, but to an extent gaffes are part of his persona and he delivered an improved performance in the first hour. Warren and Sanders, second and third in the polls respectively, old friends and targeting similar voter bases, again declined to attack one another.

2. Outsiders make a point

The debate gave an opportunity for lower-polling candidates to grasp more airtime. Former Texas congressman Beto O'Rourke, Minnesota Senator Amy Klobuchar and New Jersey Senator Cory Booker delivered strong performances.

O'Rourke, praised by other candidates for his leadership following the massacre in his hometown of El Paso, has turned gun control into his defining campaign issue. He spoke passionately about racial justice and received strong applause from the audience for his plan to buy back assault weapons. "Hell yes, we're going to take your AR15, your AK47," he said.

Klobuchar, who has struggled to gain momentum so far in the campaign, took the opportunity of being on the debate stage alongside leading candidates to offer substantive answers and outline her moderate, midwest credentials. She contrasted her positions with those of Warren and Sanders, and reignited talk that she could be the moderate undercard to Biden.

New Jersey Senator Cory Booker also delivered another upbeat performance, again trying to bring candidates together and stressing the importance of keeping the party united.

South Bend Mayor Pete Buttigieg, the only veteran on stage, enjoyed his strongest moment talking about engagement overseas, saying "the best way to avoid endless wars is not to get into them in the first place" and attacking Trump for using military personnel as "props."

3. Trade policy

In a round on trade policy several candidates made reference to the losses suffered by farmers in the midwest, an area that will be electorally pivotal in 2020, as a result of Trump's tariffs.

Klobuchar accused Trump of "treating farmers and workers like poker chips in one of his bankrupt casinos" in his tariff policies. "He has made a mockery of focused trade policy," the Minnesota senator added.

The "erratic trade war" is hurting American families, Castro argued, saying that as president he would ratchet down tensions. Trump is "conducting trade policy by tweet born out of his fragile ego," Harris said, before appearing to take a swipe at Warren by saying she was not a protectionist.

4. Healthcare divide

Healthcare was the issue that propelled the Democrats to a strong performance in the 2018 midterms, and polls suggest it remains the top issue for voters. The candidates were unified in their criticisms of Trump's approach but also had fierce disagreements between centrist and more leftist positions. 

Biden and Sanders, suffering from a cold, clashed on their respective plans. Biden aims to build incrementally on Obamacare, allowing people to keep private plans if they want to, while Sanders — whose plan is backed by Warren — is targeting Medicare-for-all with a proposal that would lead to people losing private insurance.

Warren — who decades earlier hit at Biden's ties to the financial services industry — also targeted the former vice-president for defending the interest of insurance companies, saying "I've never met anyone who likes their health insurance company."

5. Viral moments

Not many viewers will have spent three hours watching the entire debate. For all the candidates, particularly those struggling in the polls, creating a moment that goes viral on social media is now part of the debate game plan.

Castro's attack on Biden was the most talked-about moment, but California Senator Kamala Harris also provoked 'ahhs' from the audience with her line on Trump's responsibility in mass shootings by white supremacists. "Obviously he didn't pull the trigger, but he's sure been tweeting out the ammunition," she said.

Harris also referenced the Wizard of Oz when talking about Trump's approach to trade, saying "you know, when you pull back the curtain, it’s a really small dude?" evoking memories of Senator Marco Rubio's "small hands" jibe in 2016.

Booker had some lighthearted moments that quickly spread online, referencing his own baldness with a joke about Canadian Prime Minister's Justin Trudeau's hair and joking about O'Rourke's tendency to translate his answers into Spanish.