Sen. Cory Booker, D-N.J., shakes hands with former Vice President Joe Biden on Wednesday night after the second of two Democratic presidential primary debates hosted by CNN in Detroit. Paul Sancya/Associated Press

DETROIT — Democratic presidential contenders have opened a surprising new front in their effort to retake the White House – calling into question the legacy and leadership of President Barack Obama, the party’s most beloved leader.

Like young adults seeking to break away from their father’s shadow, the candidates who gathered in Detroit to debate the party’s future this week repeatedly challenged Obama’s record, both directly and indirectly, as too timid, misguided or insufficient for the moral challenge of the moment.

“It looks like one of us has learned the lessons of the past, and one of us hasn’t,” said former Obama housing secretary Julián Castro at a key moment in Wednesday’s debate, when he attacked Joe Biden, with whom he served under the former president, for refusing a more dramatic departure from his immigration approach.

The backlash Thursday was swift and furious from some close Obama allies, who expressed alarm at the prospect of turning off voters the party needs to energize for Democrats to have a chance of beating President Trump next year.

“To my fellow Democrats,” former Obama attorney general Eric Holder Jr. tweeted. “Be wary of attacking the Obama record. Build on it. Expand it. But there is little to be gained – for you or the party – by attacking a very successful and still popular Democratic President.”

Biden, who has made his eight-year partnership with Obama a centerpiece of his candidacy, sought Thursday to defend the former president while acknowledging that politics has evolved since they shared the ballot.

“I was a little surprised by how much incoming there was about Barack, about the president,” the former vice president told reporters.

“I don’t think there is anything he has to apologize for,” Biden said. “The world has changed since Obama, and here’s the deal: This is about the future. This is about taking the same sort of integrity and moving forward.”

The growing criticism of Obama underscores the leftward movement of the party in the years since he left office.

Replacing Obama’s signature accomplishment, the Affordable Care Act, has become the primary policy goal of many of the leading 2020 contenders. Several others have attacked Obama’s efforts to secure a new trade deal with Asia, his decision to surge troops into Afghanistan and the practice of courting wealthy donors, which anchored both of Obama’s campaigns for president.

“We have tried the solution of Medicare, Medicaid and private insurance,” Sen. Elizabeth Warren, D-Mass., said Tuesday night, defending her plan to replace Obama’s health reforms with a single government plan. “And what have the private insurance companies done? They’ve sucked billions of dollars out of our health care system.”

The turnabout comes as the party enters a molting period that often occurs during open presidential nomination fights. Faced with the threat of Trump’s re-election and unaddressed economic frustrations, the future of the party now appears more up for grabs than at any point since the early 1990s, when then-Gov. Bill Clinton of Arkansas emerged from the Democratic primaries by promising a new third way of political moderation.

The political messaging consensus that won Democrats control of the House in 2018 – defend Obamacare, oppose Republican policies and mostly avoid disruptive liberal ideas – also has faded over the last year as candidates try to placate this year’s crop of activists.

Democratic Party Chairman Tom Perez, another former Obama Cabinet official, has been working to tamp down the rising tensions and keep the party focused on its common enemy: Trump.

“I think we have a 100 percent values alignment,” he said when asked about the new divides. “We can debate whether we are 85 percent or 90 percent up the mountain on universal health care. Every Democrat in the field wants to get there.”

Before the debate began Wednesday, he addressed the audience at the Fox Theatre in downtown Detroit, imploring them to keep their eye on Trump and his Republican allies. “Am I the only one who misses Barack Obama in this room?” he thundered, prompting cheers.

But the unity did not continue into the main events. During Tuesday’s debate, the two leading candidates onstage, Warren and Bernie Sanders, I-Vt., both offered calls for political and economic changes far more sweeping than what Obama offered. Both campaigns are anchored in the idea that the central political crisis preceded Trump and was not solved by Obama – “a corrupt, rigged system that has helped the wealthy and the well-connected and kicked dirt in the faces of everyone else,” as Warren put it.

“There is a tension in the party now between the rank and file and a sort of rump element of establishment types who really answer to the donor class,” said Jeff Weaver, Sanders’ 2016 campaign manager, asserted regarding the emerging split.

On Wednesday, with Biden standing center stage, attacks on Obama’s record became proxies for attacking Biden’s own rationale for the race, which is anchored in his relationship with the former president.

“Barack Obama knew exactly who I was,” Biden said. “He chose me, and he said it was the best decision he made.”

But the other candidates challenged Biden for sticking too closely to the Obama script. Washington Gov. Jay Inslee bristled at Biden’s proposed climate change policy, which seeks a more gradual approach to ending dependence on coal, an echo of Obama’s embrace of “clean coal” in both of his presidential campaigns.

“The time is up,” Inslee said. “Our house is on fire.”

New York Mayor Bill de Blasio repeatedly challenged Biden to say whether he opposed Obama’s deportation policy, which expelled more than 3 million people over two terms. Biden declined to respond directly.

Within minutes of the debate’s conclusion, Biden advisers were already suggesting some of his rivals had made a mistake.

“Many people on this stage spent more time attacking Obama than they did Trump,” said Anita Dunn, a Biden adviser who previously worked for Obama. “I think Democratic primary voters will make a judgment about this.”

Without a doubt, Obama remains enormously popular in the Democratic Party. He left office with an average approval rating of 83 percent among Democrats in Gallup polls during his tenure. A Pew poll this spring found 51 percent of Democratic leaning voters believed Obama was the best president in their lifetime. Spokespeople for Obama, who has tried to stay out of the nomination fight, declined to respond to a request for comment on the controversy Thursday.

The fusillade against the former president carries particular risk of alienating black voters, a key bloc that currently backs Biden over the more-left-leaning alternatives.

“This whole suicide mission of going after Barack Obama smells like desperation, and I think it certainly shows that some of them are just not ready for where they are,” commentator and activist Al Sharpton said Thursday on MSNBC.

But even Biden has backed away from key parts of the Obama legacy, saying he would “absolutely not” continue Obama’s deportation policy, that the 2009 surge in Afghanistan that Obama supported was a “mistake” and that he would seek a new approach to negotiating the Trans Pacific Partnership, a trade deal with Asian countries that Biden had promoted as vice president. Pressed on the trade issue Wednesday night, Biden said that he would not support rejoining the pact “as it was initially put forward.”

Sen. Cory Booker, D-N.J., seized on the complexity of Biden’s message, arguing that he should be burdened by the failures of the Obama years, particularly around immigration.

“You invoke President Obama more than anybody in this campaign,” Booker said. “You can’t do it when it’s convenient and dodge it when it’s not.”

On Thursday, several Democratic candidates sought to assure voters that they could be critical of Obama, or support policies he did not, without taking away from the accomplishments of the administration.

Sen. Kamala Harris, D-Calif., cast her own support for a Medicare-for-all plan, with allowances for some private insurance, as a continuation of Obama’s legacy, not a departure from it.

“Listen, I have nothing but praise for Barack Obama,” she told reporters in Detroit on Thursday. “My proposal is about taking it to the next step.”

Booker defended the need to criticize Obama, while also praising his administration. “He ain’t perfect. Nobody’s ever pulled that off,” Booker said on CNN. “If he were running for president for a third term, I wouldn’t be running.”

Such reassurances did not assuage all Democrats who are worried about the shift in tone and strategy.

“Attacking the Obama administration is just nutty,” former senator Claire McCaskill, D-Mo., who lost re-election last year, said on MSNBC.