Joe Biden’s rivals in the race for the 2020 Democrat presidential nomination are attacking him from all angles, save one: Biden’s time as Barack Obama’s vice president.

“People are very nostalgic for that time,” an activist told Politico. Among liberal voters, the Obama administration is inextricably entwined with pre-Trump nostalgia. Years after his presidency, Obama remains extremely popular with his base. That is good news for Joe “Malarkey” Biden, who is riding that goodwill toward the Oval Office.

“It’s going to be challenging for progressives to attack that legacy,” said chief executive Yvette Simpson, of the “Democracy for America” PAC. “Because Obama not only is and was so popular, but people are very nostalgic for that time, particularly after a few years of Trump.”

Cory Booker has called a crime bill that Biden helped write in 1994 “awful” and “shameful.” Bernie Sanders has gone after Biden for his support of the Iraq War and NAFTA, while Elizabeth Warren has criticized him as “on the side of the credit card companies.” None of them, however, seem willing to contest any matter from his actual White House tenure, despite Politico noting the left has plenty of issues with the Obama administration’s legacy:

For years, left-wing activists have disapproved of the Obama administration’s management of the economic crash, opioid crisis, immigrant deportations, and ill-fated attempts to compromise with Republicans. But many believe it would be political suicide for progressive presidential candidates to question Obama’s record at length, even in the service of defeating Biden.

Sean McElwee, the co-founder of the left-wing think tank Data for Progress, had an arch response: “The biggest weaknesses Biden has, for the most part, are not things he did in the Obama administration,” he said. “Luckily for progressives, Joe Biden is literally 150 years old, which means he has a half-century of a career otherwise to attack.”

Adam Green, co-founder of Progressive Change Campaign Committee — which recently endorsed Warren over Biden — simply does not think Joe is right for the job. “It’s perfectly consistent to say that President Obama righted the ship and aimed it in a better direction,” he claimed, “but now we have an opportunity to move the ship much further and much faster toward progress.”

“The person to do that is clearly not Joe Biden,” Green added, “as he moves backwards on issues ranging from the Hyde Amendment to NAFTA to a ‘middle ground’ on the existential climate crisis.”

Meanwhile, Biden has drawn a sought-after demographic into his fold: black Americans who supported his “buddy Barack.” Yvette Simpson, head of the progressive Democracy for America PAC acknowledged the risk of alienating that demographic. “Biden’s early advantage among African-Americans has more to do with Obama than Biden. And if you attack that, you start to alienate those voters,” she said.

“Biden is winning, or at least is ahead, because nobody has made the argument that Obama’s policies are the reason that Democrats lost in 2016,” said Matt Stoller, a former Senate Budget Committee aide under Bernie Sanders. “They’re not challenging the fundamental narrative that Joe Biden is running on, which is that Obama did a good job and we need to get back to that.”

“I’ve been bugging the campaigns about it,” he said, but “they’re like, ‘Yeah, yeah, we know, but we don’t have a way to do it.'”