He might not have shared the stage with anyone this week, but former President Barack Obama was on the lips of many candidates seeking to unseat President Donald Trump in 2020.
The specter of Obama, who remains widely popular among Democrats, loomed large over the two nights of Democratic debates in Detroit this week. Still, the former president's legacy came under its strongest attack yet by members of his own party.
More so than any other candidate, former Vice President Joe Biden has capitalized on his proximity to Obama, hoping their close relationship will lead him to the presidency. Biden, already a front-runner with primary voters, has consistently referred to his role in the "Obama-Biden" administration on the campaign trail.
But rival Democrats on Wednesday night's debate stage took shots at Obama's immigration, trade and heath-care policies in an effort to score points.
Deportations, which increased significantly under the Obama administration, became a source of contention as former Housing and Urban Development Secretary Julian Castro battled with Biden over the legacy of that policy.
"It looks like one of us has learned the lessons of the past, and one of us hasn't," Castro said after Biden defended Obama's immigration record.
Protesters interrupted Biden's comments, expressing their disapproval with the 3 million deportations that happened under Obama that have led some critics to refer to him as the "deporter in chief."
During another heated exchange, New Jersey Sen. Cory Booker called out Biden for leaning selectively on his relationship with Obama. Biden had refused to answer a question on whether he supported Obama's deportation strategy as vice president, insisting he would keep those discussions private.
"Mr. Vice President, you can't have it both ways. You invoke President Obama more than anybody in this campaign," Booker said. "You can't do it when it's convenient and then dodge it when it's not."
Progressive candidates aggressively questioned Obama's centrist policies that they say are in conflict with the pulse of today's party. Even Obamacare came under attack for not covering all Americans and enriching insurance companies.
"Times change. They are not necessarily critical of Obama the person, but the policy," said Kelly Dietrich, founder and CEO of the National Democratic Training Committee. "It is politically what they need to do to create contrast in the polls with Joe Biden."
The attacks from the party's left wing came as something of a shock to Biden. "I was a little surprised at how much incoming there was about Barack, about the president," Biden said to reporters Thursday. "I'm proud of having served with him. I'm proud of the job he did. I don't think there's anything he has to apologize for and it surprised me, the degree of criticism."
Other Obama administration alumni warned that the attacks could divide the party when its focus should be on defeating Trump.
Eric Holder, who was attorney general in the Obama administration from 2009 to 2015, cautioned the Democratic field on Twitter, saying candidates should "be wary" of attacking the record of a popular president.
Not every candidate was eager to criticize the former president. Multiple candidates tried to link their ideas and policies to Obama, hoping that his high favorability would legitimize them as someone in lockstep with the needs of the Democratic Party.
"It's the best way to legitimately establish themselves as a good Democrat that shares the values that [primary voters] miss," said Gillian Rosenberg Armour, managing director at Wildfire Contact and an alum of Obama's 2008 presidential campaign.
During Tuesday night's debate, Former Maryland Rep. John Delaney expressed his support for the Trans-Pacific Partnership, Obama's signature free trade agreement. Trump withdrew from the pact in 2017 after criticism from both sides of the aisle, including Democratic candidates Hillary Clinton and Bernie Sanders.
"I think President Obama was right," Delaney said at Tuesday night's debate in Detroit. "We would be in an entirely different position with China if we entered the Trans-Pacific Partnership. We can't isolate ourselves from the world."
Montana Gov. Steve Bullock defended the Obama administration's opposition to decriminalizing unauthorized border crossings, while more liberal members argued the move would alleviate stress on a beleaguered immigration system.
"We've got 100,000 people showing up at the border right now. If we decriminalize entry, If we give health care to everyone, we will have multiples of that," Bullock said during the debate. "Don't take my word, that was President Obama's Homeland Security secretary who said that."
The robust debate may make good television, but some strategists fear the intraparty sniping could threaten its general election chances.
"We need to be careful as a party that we don't get too far into mud that we can't come back and unify," says strategist Andrew Feldman who was political director on John Delaney's 2012 congressional campaign. "The ultimate goal is winning the White House."