(CNN) Joe Biden is defending what many Democrats see as all but indefensible: the 1994 crime bill .

At virtually every campaign stop, the former vice president fields questions about the consequences of the measure, which was signed into law by President Bill Clinton 25 years ago and is now a potential weight around Biden's 2020 presidential bid. It's the starkest example of how a signature piece of his legislation, which most Democrats supported at the time, is viewed in a far different light today.

Inside Biden's camp, some aides believe he must do more to accept the consequences of the law, as Clinton and many supporters of the bill have done. But Biden has been insistent on defending what he believes were good portions of the bill that led to a drop in crime across the country.

Asked about prison reform this week in New Hampshire, Biden brought up the crime bill. He told the audience member who asked about prison reform "you've been conditioned to say" that the 1994 legislation "is a bad bill."

He said there is "only one provision in there that had to do with mandatory sentences that I opposed. And that was a thing called the 'three strikes and you're out,' which I thought was a mistake. But had a lot of the good things in the bill."

As he takes questions from voters, it often takes Biden several minutes to explain the provisions in the bill, insisting it did not lead to mass incarceration.

"This idea that the crime bill generated mass incarceration -- it did not generate mass incarceration," Biden said in an earlier swing through New Hampshire in May.

Several of his rivals have rushed to disagree, teeing up a likely conflict at the first Democratic debate in just three weeks.

"That 1994 crime bill -- it did contribute to mass incarceration in our country," California Sen. Kamala Harris said recently. "It encouraged and was the first time that we had a federal three-strikes law. It funded the building of more prisons in the states."

"That bill was awful," New Jersey Sen. Cory Booker said last week in an interview with the Huffington Post -- after saying, "I love Joe Biden."

"Good people signed on to that bill. People make mistakes," Booker said. "But let's hold them to that. That crime bill was shameful, what it did to black and brown communities like mine (and) low-income communities from Appalachia to rural Iowa. It was a bad bill."

President Donald Trump is also using the issue to mount an attack against Biden that appears more likely to prove damaging in the Democratic primary than in the general election.

"Anyone associated with the 1994 Crime Bill will not have a chance of being elected. In particular, African Americans will not be able to vote for you. I, on the other hand, was responsible for Criminal Justice Reform, which had tremendous support, & helped fix the bad 1994 Bill!" Trump tweeted in late May.

The President is unlikely to win significant support from black voters in 2020 given his own past actions -- including leading the false birtherism conspiracy theory against former President Barack Obama and taking out a full-page newspaper ad in 1989 calling for the execution of the "Central Park Five," black teenagers who were wrongfully convicted of a brutal rape but later exonerated.

But his focus on the crime bill could increase attention on an issue with the potential to drive a wedge between Biden and black voters; polls have shown that bloc is his most significant base of support in the 2020 Democratic nominating contest -- particularly in South Carolina, an early-voting state where the majority of the Democratic electorate is African American.

Once supported, now criticized

The bill, formally known as the Violent Crime Control and Law Enforcement Act, is now roundly criticized by nearly all Democrats and some Republicans and blamed for locking up a generation of non-violent offenders.

But it passed in 1994 as a compromise. It gained support from most members of the Congressional Black Caucus as well as several big-city black mayors and clergy leaders who were alarmed at the soaring crime rates.

Biden, then the chairman of the Senate Judiciary Committee, took ownership of the measure at the time.

"A guy named Biden wrote that bill and he wrote that bill by going down and sitting down with the president of the United States of America," he said during a 1993 floor debate.

The measure was a sweeping $30 billion package, which put 100,000 new police officers on the streets, created the Violence Against Women Act and an assault weapons ban.

It also expanded the federal death penalty and created dramatically harsher sentencing laws, including three strikes: mandatory life terms for people with at least three federal violent crime or drug convictions. One of the most controversial aspects of the crime bill: granting states billions to build prisons if they passed their own tough sentencing laws with mandatory minimums.

But those state laws -- incentivized by the federal government -- are what critics say helped contribute to mass incarceration.

"In that bill, there were longer sentences. And most of these people are in prison under state law, but the federal law set a trend," Bill Clinton said in 2015. "And that was overdone. We were wrong about that. That percentage of it, we were wrong about."

"The good news is we had the biggest drop in crime in history. The bad news is we had a lot of people who were locked up, who were minor actors, for way too long," Clinton said.

Michael Waldman, president of the non-partisan policy institute Brennan Center for Justice at New York University School of Law, said the debate is notable considering it is coming a quarter-century after the bill was signed into law.

"It's striking to have a debate in 2019 be about the 1994 crime bill," said Waldman, who worked as a speechwriter to Clinton during the period. "Explaining the past to the present is never easy and not where you want to spend your time."

Waldman, who is watching the debate with interest, said he wondered why Biden has fiercely defended the bill. Clinton, for example, apologized for the fallout four years ago in the middle of Hillary Clinton's campaign, saying at a 2015 NAACP convention in Philadelphia he "signed a bill that made the problem worse, and I want to admit it."

"I would encourage him to take a more textured response to it," Waldman said of Biden. "Some things that the bill did were good and some had lasting bad consequences."