Historically, when a former vice president runs for a promotion to the top job, he struggles with the legacy of his former boss. George H.W. Bush was caricatured as a wimp as he tried to step out of Ronald Reagan’s shadow. Al Gore was paralyzed as he tried to embrace Bill Clinton’s popular policies while distancing himself from the sex scandal that led to Clinton’s impeachment. Hillary Clinton faced a similar dynamic in 2008 and 2016, when the former first lady treated her husband’s administration like a buffet, endorsing some of his policies and discarding others.

So far Biden has had it easier. Barack Obama is the most popular figure in the Democratic Party and Biden embraces the man and his legacy with the kind of affection he previously reserved only for Amtrak or Corvettes. Biden’s campaign message, as a popular joke goes, is often little more than a subject, a verb and Obama’s name. There is none of the psychodrama that consumed Gore. There is none of the picking and choosing that defined Hillary Clinton.

On Thursday night, when Biden momentarily seemed to slip and suggest that he was less than full-throated in his embrace of Obamaism, specifically about their record on immigration, Julián Castro pounced.

“My problem with Vice President Biden — and Cory pointed this out last time — is every time something good about Barack Obama comes up,” said Castro, who began to mimic Biden, “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.’ I mean, he wants to take credit for Obama's work, but not have to answer to any questions.”

Biden took the attack as an opportunity to clarify that his loyalty to Obama is unconditional: “I stand with Barack Obama all eight years—good, bad and indifferent.” That might as well be the Biden campaign slogan.

This strategy, which is often mocked, of hugging Obama like a life preserver through the rapids of the primary has carried Biden far. And Thursday night suggested that trying to break loose Biden’s grip from his Obama raft is still surprisingly difficult for his opponents.

Before the debate, one top Biden adviser predicted gleefully that there was little percentage for the other candidates in trying to attack aspects of the Obama legacy, even the parts where many on the left believe he wasn’t bold enough. “People started down the road where they were going to separate themselves from Obama,” he told me, “and they immediately understood that that is a dead end in the Democratic Party.”

The Houston debate offered a great deal of support for this view. None of the candidates has figured out a way to praise Obama and attack Biden.

It started with health care. Biden himself brought up the issue. “I know that the senator says she's for Bernie,” he said about Elizabeth Warren’s support for Sanders’ “Medicare for All” proposal. “Well, I'm for Barack. I think Obamacare worked.”

In response Warren, notably, did not say that Obama wasn’t bold enough when it came to health care, an argument that Sanders supporters and others on the left often make. Instead, she argued that she just wanted to build on Obama’s good work.

“We all owe a huge debt to President Obama,” Warren said, polishing the former president’s halo in Biden-like fashion, “who fundamentally transformed health care in America and committed this country to health care for every human being. And now the question is: How best can we improve on it?”

Kamala Harris said much of the same thing. “I want to give credit to Barack Obama for bringing us this far,” she gushed. “We wouldn't be here if he didn't have the courage, the talent to see us this far.”

Castro took it a step further, trying to steal for himself the mantle of Obama on the issue by arguing that Biden’s views on health care are actually a betrayal of Saint Obama.

“Barack Obama's vision was not to leave 10 million people uncovered,” Castro said. “He wanted every single person in this country covered. My plan would do that. Your plan would not. I’m fulfilling the legacy of Barack Obama and you're not.”

Biden, who was sharper and more energetic than the listless person who showed up in the first two debates, had a stinging retort that highlighted the fact that he knows the former president better than anyone else on the stage: “That will be a surprise to him.”

Julián’s brother, Rep. Joaquin Castro, told me after the debate that despite his popularity, Democrats needed to be realistic about Obama’s legacy. “All of us agree that President Obama was a great president,” he said. “But we can't make our presidents into flawless heroes.” So far this is a minority view.

“People are being very careful,” David Axelrod, Obama’s former chief strategist, texted me during an exchange about trade when I asked about the seeming prohibition on attacking Obama.

By the end of the night, nobody on stage had figured out how to pick the Obama-Biden lock.

“President Obama is a towering force in the Democratic Party precisely because of the contrast he provides to Trump, who serves as a constant reminder of the strengths of Obama to every Democrat in the country,” said Neera Tanden, president of the Center for American Progress and a former Clinton and Obama official. “Attacks on Obama backfired in the second debate and I think candidates were wary of delivering them again precisely because they engendered such anger before. While primary opponents can take on Biden's particular record during his service in the Obama administration, it then provides Biden the opportunity to remind people that Obama trusted Biden and he was a partner in the administration. So there are a lot of pitfalls and few rewards for any Democrat to criticize the Obama administration and its record.”

Jon Favreau, Obama’s longtime speechwriter, told me he didn’t think it should be such a challenge. “I’m a hopelessly biased defender of my former boss, but I thought the narrative that the candidates were trashing Obama’s legacy in the last debate was way overblown,” he said. “I don’t see any problem at all with candidates saying, ‘I think Obama was the best president of our lifetime, but I disagreed with how he handled deportations,’ or ‘I didn’t support the TPP’ or whatever. And I’m still waiting for one of the Medicare for All candidates to point out that one person who called their plan a good idea was … Barack Obama! (in 2018).”

He added, “So yeah, it seems like a struggle, but I’m not sure why it should be.”

But it was. Harris seemed to sum up the view of the non-Biden candidates on this topic. When asked how her views on trade differed from Obama’s, she was clear and succinct that she had no interest in messing with him.

“I have no criticism of that,” she said.