The crime bill that former Vice President Joe Biden helped write as a senator 25 years ago has been thrust into the spotlight on the 2020 campaign trail.

His primary competition, including Sens. Kamala Harris, D-Calif., and Cory Booker, D-N.J., have condemned the legislation, signed into law by President Bill Clinton, as having contributed to mass incarceration. And over the Memorial Day weekend, President Donald Trump attacked Biden for his involvement in it.

Here's what was in the bill, who supported it, and what is known about the law's effects.

What is the 1994 crime bill?

The Violent Crime Control and Law Enforcement Act of 1994, or the 1994 crime bill as it became known, earmarked billions in funding for states to build new prisons, train and hire additional police, expanded the federal death penalty and instituted a federal "three-strikes" life sentence mandate.

The legislation also included the original Violence Against Women Act, the Federal Assault Weapons Ban and more than $6 billion in funding for crime prevention programs.

The bill was intended to combat and deter violent crime, which peaked nationally in the early 1990s. On a political level, Clinton and Democrats hoped that passage of a tough law would help them blunt or reverse long-running GOP charges that their party was soft on crime.

Violent crime started to fall before the bill was passed, though it dropped much more dramatically from 1994 to 2000, according to the Brennan Center for Justice, a progressive law and policy institute.

Who was for it, who was against?

Both Democrats and Republicans supported the bill, though there were detractors in both parties.

Democrats, including most of the Congressional Black Caucus, Republicans and substantial numbers of African American pastors and mayors backed the legislation, according to reports at the time.

Most supporters saw it as a way to respond to the peak in violent crime in the early 1990s that came on the heels of a decades-long increase, a trend that would soon begin to reverse.

"The American people have been looking forward to this day for a long time," Clinton said at a bill-signing ceremony in 1994. "In the last 25 years, half a million Americans have been killed by other Americans. For 25 years, crime has been a hot political issue used too often to divide us while the system makes excuses for not punishing criminals and doing the job."

Biden had been working with Republicans on the Judiciary Committee, including Sen. Strom Thurmond, R-S.C., on similar legislation even before Clinton took office.

When the Senate passed its version of the bill in November 1993, on a 95-4 roll call, former Sen. Orrin Hatch, R-Utah, boasted about it.

"I believe the American people are starting to see that we mean business about crime, and that this bill is going to make a difference," he said then. "As in all bills, there may be some things that I do not particularly like. But, overall, this bill is a tremendous addition to the fight against crime. It is, I think, the finest anti-crime bill in the history of this country."

Current Senate Majority Leader Mitch McConnell, R-Ky., was among those who voted for it.

But there was a war brewing in the House, in which Republicans were concerned over whether the bill was tough enough on offenders, whether it spent too much on crime-prevention programs rather than punishment — the "midnight basketball" program became a talking point for conservative commentator Rush Limbaugh — and whether it should include a ban on certain semiautomatic weapons. Some centrist Democrats shared these objections.

A House-Senate conference committee had to write two different "final" versions of the legislation before it could pass muster, and an unlikely coalition of Republican and Democratic opponents temporarily killed it in the House by winning on a procedural vote. Clinton agreed to demands to trim back spending for prevention programs and tweak the weapons ban to secure the votes he needed in the House.

In the end, 235 House members, including 46 Republicans, voted for it. (Presidential hopeful Bernie Sanders, then an independent representative from Vermont, was among the "yes" votes.)

But with the National Rifle Association actively engaged against the bill, and focusing its arguments on aspects other than gun control, it suddenly became a much tougher vote for GOP senators. Biden and his allies needed 60 votes in the Senate to overcome a procedural block, which meant they had to get at least six Republicans on board. They got seven and the bill went to Clinton for his signature. McConnell voted against it the second time around.

Critics found fault

Detractors at the time warned that the bill co-written by Biden was far too punitive and did not put enough of an emphasis on dealing with broader structural issues that led to the increase in violent crime. Though there was money allocated for preventative programs, the bill was focused on cracking down on crime and throwing the book at criminals.

There were alternative legislative efforts. Then-Rep. John Conyers, D-Mich., said that providing billions for prison expansion was a "simplistic approach to the crime problem." Conyers offered up a bill that focused on job opportunities, drug treatment and additional crime prevention programs.

Meanwhile, Rep. Bobby Scott, D-Va., one of the Congressional Black Caucus members who voted against the crime bill, said, "You wouldn't ask an opponent of abortion to look at a bill with the greatest expansion of abortion in the history of the United States, and argue that he ought to vote for it because it's got some highway funding in it."

What Biden says now

Biden has faced questions and criticism about the legislation this year, before and after announcing his bid to challenge Trump in 2020. He's mostly defended the law and taken credit for some of its successes.

"You know I've been in this fight for a long time," Biden said in January at a Martin Luther King Jr. Day breakfast in Washington, without explicitly remarking on the 1994 bill. "I haven't always been right, I know we haven't always done things the right way. But I've always tried."

At a New Hampshire campaign stop in May, Biden talked up several specific components of the bill.

"There's a whole lot of talk about, you know," Biden said. "It had three big things in it," he continued, listing off the funding that was allocated toward crime prevention programs, as well as the Violence Against Women Act and the ban on assault weapons, both of which are now expired.

Sanders also has defended his vote in favor of the bill in a similar fashion, telling CNN that if he hadn't, he would be getting asked today about why he "did not vote for a ban on assault weapons."

Did it lead to "mass incarceration"?

During that same New Hampshire event, Biden claimed that the 1994 bill did not lead to mass incarceration.

"Folks, let's get something straight, 92 out of every 100 prisoners who end up behind bars are in a state prison, not a federal prison," he said. "This idea that the crime bill generated mass incarceration, it did not generate mass incarceration."

Though the bill was not the root cause of "mass incarceration," it was "the most high-profile legislation to increase the number of people behind bars," according to a Brennan Center analysis in 2016.

The crime bill granted states billions to build prisons if they passed laws requiring inmates to serve at least 85 percent of their sentences, the Brennan Center said, noting that 30 states introduced or amended laws between 1995 and 1999 so that they would be in compliance and receive the money. By 1999, 42 states had "truth-in-sentencing" laws on the books, which contributed to an increase in imprisonment.

"By dangling bonus dollars, the crime bill encouraged states to remain on their tough-on-crime course," the Brennan Center wrote.