Julián Castro, the lowest-polling candidate in the Democratic debate Thursday night, reached up the ladder and tried to yank Joe Biden off it.

Castro’s decision to attack the former vice president and 2020 front-runner, in a way that appeared designed to highlight Biden’s advancing age, was a high-stakes move at a time when the former Housing and Urban Development secretary needs a big boost of momentum to change the trajectory of his campaign.

But it may well backfire, inspiring a shocked reaction from Democrats in the audience and fact-checking of Castro’s specifics afterward — the latest in a series of moves against Biden that, in retrospect, are looking more like cautionary tales.

Try as they may, nobody has been able to jolt Biden off of his polling lead in the Democratic primary — at least not yet. Not Kamala Harris, who did rattle him in the first debate, only to see all of her gains slip away during a lost summer for her campaign. Not Cory Booker, who took on Biden over criminal justice issues later this summer, to no measurable bounce. And certainly not Eric Swalwell, who quickly dropped out of the race after imploring the former vice president to “pass the torch.”

The past back and forths seemed to inform the strategies of Biden’s higher-polling rivals on Thursday: While Biden was an early aggressor, Bernie Sanders and Elizabeth Warren broke with him over the policy but generally declined to offer sharp rejoinders or jump in to attack him.

But about a half hour into this latest debate, as an early series of questions about health care drilled deep into the candidates’ specific positions, Castro leapt at a chance to contrast his plan with Biden’s.

The difference between what he supports, and what Biden backs, Castro teased, “is that you require [Americans] to opt in” to a Medicare-like public health insurance option in certain scenarios, like job loss. “And I would not require them to opt in — they would automatically be enrolled.”

With the debate audience watching him, Castro then needled Biden by invoking Barack Obama, arguing that the former president — and Biden’s much-invoked former boss and ticket-mate — did not intend to leave 10 million people without health care coverage. “He wanted every single person in this country covered,” Castro added, with another punch. “My plan would do that. Your plan would not.”

Biden was ready with a real-time fact-check, suggesting he’d learned from the past exchanges. He was dismissive, but imprecise in his pushback.

“They do not have to buy in. They do not have to buy in,” Biden insisted, as Castro laid in wait.

“You just said that two minutes ago. You just [said] two minutes ago they would have to buy in,” he responded. Then, he launched into an unmistakable brushback based on Biden’s age.

“Are you forgetting what you said two minutes ago?

“Are you forgetting already what you said just two minutes ago?”

Castro paused as the crowd let out a Jerry Springer Show-like round of “ohhhhs.”

“I can't believe that you said two minutes ago that they had to buy in and now you're saying they don't have to buy in,” Castro continued, to the crowd’s shock. “You're forgetting that?”

Biden wasn’t having it. He looked disgusted with the exchange, turning away from Castro.

But the Texan wouldn’t relent. Castro ticked though more details and claimed that he, in fact, was the one fulfilling the legacy of Obama — "and you're not.”

“That will be a surprise to him,” Biden finally shot back.

It wasn’t the only time Castro went on attack. Later in the debate, as the conversation turned to immigration, he slammed Biden for trying to take credit for the positive aspects of Obama’s record, and shying away from the unfavorable parts.

“Every time something good about Barack Obama comes up, he says, “Oh, I was there, I was there, I was there, that's me, too,’ and then every time somebody questions part of the administration that we were both part of, he says, ‘well, that was the president,’” Castro said.

He even poked at Biden for droning on too long in a later answer.

But it was the attack over health care that stood out, illustrating the high stakes and dwindling time for low-polling candidates, who are now having to consider riskier tactics.

In this case, it’s not clear if Castro’s gambit will wear well. Minutes before Castro launched his inquiry, Biden claimed of his plan that it would do the opposite of what Castro claimed. Castro may have been right about the number of people Biden’s plan would leave uncovered, but his repetitive quip failed to capture what Biden had actually said moments earlier.

“Anyone who can’t afford it gets automatically enrolled in the Medicare-type option we have,” Biden had said. He had added later, “You can automatically buy into this.”

Castro’s campaign has pointed to language on Biden’s campaign website suggesting that enrollment in a public health care option would hinge on people interacting with other pieces of the machinery of government to trigger the enrollment. And Castro stood by his approach in a post-debate interview on CNN.

“It was not intended as a personal attack or affront,” Castro said.

But he added, “This is a debate. And when we're talking about health care policy, we're talking about a policy that impacts every single person.”

"I hear you,” replied CNN anchor Chris Cuomo. “But this show's all about robust debate and it's also about decency. It seemed like a cheap shot.”

“Not at all,” Castro said. “Not at all.”