HOUSTON — Vice President Joe Biden, the front-runner standing center stage at Thursday night’s Democratic presidential debate, knew he would be the prime target of the nine other candidates who can’t rise unless he falters, and so he struck first, on health care, perhaps the central issue of the 2020 campaign.

Noting that Massachusetts’ Sen. Elizabeth Warren, who is now seen as Biden’s rising rival, backs Medicare for All along with Vermont's Sen. Bernie Sanders, but Biden supports expanding Obamacare to include a public option, which Biden compactly reduced to: “She’s for Bernie; I’m for Barack.”

Warren repeated her calls for Medicare for All.

“I’ve never actually met anyone who likes their health insurance carrier,” she said. “What they want is access to health care.”

“What people want is cost-effective health care,” Sanders said.

And, while Sanders repeated his signature comment about Medicare for All — “I wrote the damn bill” — Minnesota's Sen. Amy Klobuchar, who presented herself as a moderate alternative for people tired of the extremes and “the noise” and does not support the Sanders plan, said, “While Bernie wrote the bill, I read the bill.”

“Medicare for All is comprehensive health care,” Sanders replied. “It allows you to go to any doctor you want.”

But, while both Warren and Sanders fired back, the sharpest shot at Biden came from the far end of the lectern in the person of Julián Castro, the former mayor of San Antonio and Obama administration housing secretary.

“I also worked for President Obama,” he reminded Biden. “I want every single American family to have a strong Medicare plan available if they choose to hold on to strong, solid private health insurance.”

“Are you forgetting what you said two minutes ago?” Castro asked Biden. “I’m fulfilling the legacy of Barack Obama, and you’re not.”

Castro kept at it, pressing Biden on whether he was forgetting what he had just said.

“This is why these debates have become unwatchable,” said South Bend, Ind., Mayor Buttigieg, warning against self-destructive backbiting.

But Castro said the give-and-take is what an election is all about.

Thursday night was the first time the party’s top 10 candidates, as measured by poll standing and breadth of fundraising, all shared the same stage at the same time, their standing in the polls represented by their array on the stage, with the front-runners center stage and radiating out to the longer-shot candidates in the wings.

The debate had opened with Castro welcoming the national audience: “Bienvenidos a Tejas, welcome to Texas.”

Castro pitched the 2020 election as a battle for the future.

“There will be a life after Donald Trump,” Castro said. “But first we have to win."

"That means exciting a young, diverse coalition of American who are ready for the future,” Castro said. “It’s what I can do in this race. Get back Michigan, Wisconsin and Pennsylvania, Florida, Georgia and Arizona and finally turn Texas blue and say goodbye to Donald Trump.”

The shape of the race, less than five months out from the first voting in Iowa on Feb. 3, seems pretty clear. Biden, who has run for president twice before, has held a healthy but not commanding lead throughout the process. But at 76 and with a lifelong tendency toward garrulousness that intermittently crosses the line into gaffes, he has left questions about whether he is up to the task. Between them, Sanders and Warren, the leading apostles of the progressive agenda and who are within a point or two of each other, could rival Biden in the polls.

Beto O'Rourke, whose hometown of El Paso has dealt with the aftermath of the Aug. 3 mass shooting that left 22 people dead and 26 injured at a Walmart, used his opening statement to talk about the shooting and his call for gun control.

“Racism and violence that has long been a part of America was welcomed out into the open and directed to my hometown,” he said, adding that politics have been insufficient to similar attacks. “We have to see clearly, we have to speak honestly.”

Entrepreneur Andrew Yang used his opening statement to deliver on a promised surprise at the debate. He announced that his campaign would give 10 families $1,000 a month as a pilot program for his universal basic income proposal.

“This is how we will get our country working for us again, the American people,” Yang said.

Under Yang’s proposal, which he calls the Freedom Dividend, every American over 18 would receive a payment from the government for $1,000 a month, or $12,000 a year.