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Lanyards for members of the media sit ahead of the Democratic presidential candidate debate in Houston on Thursday.

The third presidential debate threw into sharp relief a dividing line among Democrats, as the contenders argued how far to go to insure more Americans, remove guns from the streets and promote fair trade.

Over three hours, former Vice President Joe Biden led the moderate camp, raising doubts about the cost of Medicare for All and defending his four-decade record of public service.

In her first time on stage with Biden, Massachusetts Senator Elizabeth Warren held back, training her fire on Wall Street and Washington lobbyists rather than her Democratic rivals and retelling her personal story.

But with Warren rising in the polls, Biden took the chance to press her on her health care proposal.

"Thus far, my distinguished friend, the senator on my left, has not indicated how she pays for it," he said.

The rest of the field had brief moments that, for better or worse, defined their nights. California Sen. Kamala Harris used her time to directly attack President Donald Trump; former Housing and Urban Development Secretary Julián Castro went after Biden on health care and immigration; South Bend, Indiana, Mayor Pete Buttigieg popped in at times to call for civility and former Texas representative Beto O'Rourke repeated his call for a mandatory assault weapon buyback.

Entrepreneur Andrew Yang announced that his campaign would give $1,000 a month for a year to 10 families to highlight his proposal for a universal basic income.

There were likely no game-changing moments during the debate, but it provided plenty of fodder for moderates and progressives as they outline two very different approaches to taking on Trump in the months to come.

Exposing a longstanding rift on trade: The Democrats' traditional disagreements on trade -- protectionism versus free trade - broke through after an initial show of unity over the candidates' mutual disdain for Trump's negotiation-by-tweet.

But they didn't answer the question of whether they'd remove his tariffs immediately. Nor did they have a detailed exit strategy for the trade war.

The main consensus was that the next Democratic president would seek to tone down the conflict, using the existing tariffs as leverage. Bernie Sanders and Warren argued that U.S. trade policy has been driven too much by corporate interests, saying they'd bring labor unions, environmentalists, human rights activists and small farmers to the negotiating table with them.

"I would immediately begin to negotiate with China to ratchet down that trade war," Castro said. "We have leverage there."

Sanders criticized Trump for setting trade policy via 3 a.m. tweets, while Buttigieg argued that the current administration lacks a strategy.

"We can use trade to help build a stronger economy," Warren said.

• Fixed positions on health care: The contours of the Democratic debate over health care didn't change Thursday night, but the temperature did, as the familiar split between moderates and progressives intensified.

Biden attacked the Medicare-for-All proposal supported by Sanders and Warren, arguing that it would force people off their health plans and raise taxes on the middle class. The onslaught borrowed a strategy that Republicans once wielded against the Affordable Care Act, or Obamacare.

Biden wants to build on the Affordable Care Act by offering people the option to join a public plan. He also echoed a promise by Barack Obama that came back to haunt the former president. "Of the 160 million people who like their health care now, they can keep it. If they don't like it, they can leave," Biden said.

Sanders and Warren countered that Medicare-for-All would relieve Americans burdened by rising medical costs, and rely on taxes on the wealthy and large corporations to fund it. "Those at the very top, the richest individuals and the biggest corporations, are going to pay more," Warren said. "And middle class families are going to pay less."

Castro sharply criticized Biden, challenging the former vice president's attempt to claim the legacy of Obama. Castro, like Biden, favors preserving private insurance and adding a public option, but he said his policy would automatically enroll the uninsured, covering more people.

• O'Rourke has a moment on guns: Gun control brought impassioned pleas from several candidates. O'Rourke received praise from his rivals for how he responded to the shooting last month in his hometown of El Paso, Texas, that killed 22 people and wounded 24. His proposal to enforce a mandatory gun buyback program for military-style automatic weapons received loud cheers from the audience.

When asked whether the deadliest types of guns should be confiscated, O'Rourke said, "Hell, yes, we're going to take your AR-15, your AK-47." He then described talking to victims of the shooting and accused Trump of inspiring the attack.

O'Rourke paused his presidential run to return to El Paso. Since then, he's defined himself as the gun control candidate, expressing outrage at the lack of action in Washington and vowing to act quickly on the hot-button issue if elected.

While Cory Booker acknowledged O'Rourke's response to the shootings, he suggested O'Rourke was a late comer to the problem of guns.

"I'm sorry that it had to take issues coming to my neighborhood or personally affecting Beto to suddenly make us demand change," Booker said. "This is a crisis of empathy in our nation. We are never going to solve this crisis if we have to wait for it to personally affect us or our neighborhood or our community before we demand action."

• Biden, Castro clash: One of the harshest moments of the debate was a clash between two members of the Obama administration, Biden and Castro, over a minor technical issue.

Seeking to claim Obama's mantle, Castro argued that his health care plan was better than Biden's because people who qualified would automatically be enrolled, arguing that they would have to opt in to Biden's Medicare for Choice plan.

"They do not have to buy in," Biden said.

In fact, Biden had said earlier that people who couldn't afford insurance would be "automatically enrolled in the Medicare-type option that we have."

But Castro seized on the disagreement to raise doubts about Biden's mental acuity.

"You said they would have to buy in. Are you forgetting what you said two minutes ago?" he said.

Castro then invoked their former boss. "I'm fulfilling the legacy of Barack Obama, and you're not," he said.

Biden shot back: "That'll be a surprise to him."

• Harris, Buttigieg can't break through: Stuck in the mid-single digits in polls, Harris and Buttigieg needed the debate to give them a boost. Instead, they both largely faded into the background.

Harris opened by directly addressing the common enemy of all of the candidates: Trump. She said she wanted to deal with the president before spelling out her policy proposals.

"But first, I have a few words for Donald Trump, who we all know is watching," Harris said. "So, President Trump, you spent the last two and a half years full-time trying to sow hate and division among us. And that is why we've gotten nothing done."

Trump however was not watching at that moment. He was giving a speech in Baltimore.

Buttigieg highlighted his plan to address racism, he questioned Sanders and Warren on whether they trusted Americans to make decisions with their health care, and talked about his military service in Afghanistan. But he was unable to deliver a sharp definition of his candidacy or the qualities that made him stand out from his rivals.

Harris, too, failed to grab the spotlight. She couldn't replicate the buzz generated by her clash with Biden over race at the first debate. She garnered applause when she said that while Trump hadn't pulled the trigger in the El Paso massacre last month, he had been "tweeting out the ammunition." Otherwise, she offered only a tepid defense of her record as a prosecutor, and lacked passion even when discussing her signature issue of raising teacher pay.

• Biden stands up for pragmatism: Biden has centered his campaign on taking a pragmatic and more moderate approach than many of his Democratic competitors. And from the first moments of Thursday's debate, he drew a sharp contrast between himself and the most progressive candidates, Warren and Sanders.

"Now, it's not a bad idea if you like it," Biden said about Medicare for All. "I don't like it."

The debate over health care - more than any other issue - revealed the clear fissures in the Democratic contest. Biden found support for his centrist politics from Sen. Amy Klobuchar of Minnesota and Buttigieg. In particular, Biden pushed his left-leaning rivals to answer for how they would pay for their expansive policy proposals.

As the nominating contest hurtles into the fall, the debate showed voters the two camps that candidates fall into: Those who are calling for substantial change to the political and economic system and those who are calling for more moderate, incremental policy positions.

The leading candidates in those two camps, however, never really took shots at one another. With Biden and Warren sharing the debate stage for the first time, pundits hoped for a confrontation between the two. The sparks, however, never flew. They disagreed on the policy details, but the conversation stayed civil. It was the lowest polling candidates, in particular Castro, who took the shots at Biden.

• Yang's money giveaway: After chiding the moderators of the first two debates for giving him less speaking time than the other candidates, Yang made sure to make use of his time in Houston.

The day before the debate, his campaign manager let drop that the candidate intended to do "something no presidential candidate has ever done before in history." In his opening statement Yang announced he would be giving out his signature "freedom dividend" -- $1,000 for every American over the age of 18 -- to 10 families for a year, as a kind of down payment on his promise to guarantee every American a universal basic income if he is elected.

The stunt led to questions over its legality. In a release the campaign said it consulted his counsel and found that the donation would pass muster under campaign finance laws.

Yang has been able to build a fervent online following that has allowed him to qualify for debate stage when established politicians such as Senator Kirsten Gillibrand of New York could not. This round, he seemed to be self-deprecating when he said: "I'm Asian, so I know a lot of doctors."

By the end of the debate, Yang had gained more than 7,000 Twitter followers, the most of any candidate on stage.