Although Joe Biden has not officially entered the Democratic primary, early polls suggest he’s well-liked. One recent CNN survey found that Biden is the most popular presidential hopeful. (This early on in the race, though, that may have more to do with name recognition than anything else.)

But if and when Biden really begins running, he’ll have a major liability: his record on criminal justice.

During Biden’s congressional career, he pushed several pieces of legislation that lengthened criminal sentences, particularly for drugs favored by nonwhite users. That legislation helped create America’s dismal mass incarceration crisis. The United States has one of the largest prison populations in the world, and people of color are disproportionately impacted.

That’s got some Democrats pushing back against Biden as a candidate. As journalist Glenn Greenwald tweeted Monday:

“When someone runs for President, the lives they destroyed & the evils they ushered in when as a US Senator are relevant. And if he wants to claim he changed, he should be able to point to efforts to undo his harm: such as bills to liberalize drug laws & the Drug War.”

Biden’s past work on criminal justice was significant.

In 1988, then a senator from Delaware, Biden co-sponsored the Anti-Drug Abuse Act. That measure created new mandatory minimum sentences for drugs, and included a much-criticized sentencing rule that effectively treated possession of crack cocaine much more harshly than the possession of powder cocaine. Though the substances are nearly chemically identical, crack cocaine was more likely to be used by lower-income people, including blacks and Latinos, while powdered cocaine was more popular with people who were affluent.

Six years later, Biden led the effort to pass the 1994 Violent Crime Control and Law Enforcement Act, a “sweeping, bipartisan bill that touched nearly every aspect of American law enforcement that was signed into law by President Bill Clinton,” according to the New York Times.

Taken together, those pieces of legislation reshaped America’s criminal justice system.

“There’s a tendency now to talk about Joe Biden as the sort of affable if inappropriate uncle, as loudmouth and silly,” sociologist Naomi Murakawa, author of “The First Civil Right: How Liberals Built Prison America,” told the Marshall Project in 2015. “But he’s actually done really deeply disturbing, dangerous reforms that have made the criminal justice system more lethal and just bigger.”

As he feels out whether to run, Biden has already begun to try to contextualize those past decisions.

At a breakfast commemorating the birthday of civil rights activist Martin Luther King Jr. on Monday, Biden acknowledged the role he played in passing legislation in the 1980s that toughened sentences for drug possession.

“It was a big mistake when it was made. We thought, we were told by the experts, that crack — you never go back; it was somehow fundamentally different. It’s not different,” he said. “But it’s trapped an entire generation.” He “may not have always gotten things right,” he told the crowd.

The former lawmaker has also begun highlighting the work he did during the Obama administration to address the sentencing disparities for crack vs. powder cocaine. And he’s begun to frame the drug conversation around larger questions of race and white supremacy. “White America has to admit there’s still a systematic racism,” he said at the Monday breakfast. “And it goes almost unnoticed by so many of us.”

It’s not clear whether these efforts will be enough. Hillary Clinton struggled with her own role in the 1994 crime bill when she was running for president in 2016. Though Clinton was not a lawmaker at the time, she backed the bill with enthusiasm as first lady. In so doing, she released statements characterizing some low-income youths of color as “super predators.” Her position alienated young black voters.

(It’s worth noting, too, that Biden isn’t the only Democratic presidential candidate who will face questions over criminal justice: Sen. Kamala D. Harris (D-Calif.) has been accused of opposing reforms as California’s attorney general.)

Any change in approach to presidential hopefuls given their past on criminal justice will highly depend on them admitting that they were wrong first and explain in detail how they will work to reverse that error. If Biden officially enters the race, more than quite a few voters in the Democratic Party base will be waiting to hear from him.