Unintended consequences are a constant in politics, and many veteran Democrats talk about the prison boom of the late 20th century as if it's an example of them. Speaking about the '94 crime bill last May, for example, Bill Clinton said, "The problem [with] the way it was written and implemented is we cast too wide a net and we had too many people in prison," as if they'd intended to fight crime but hadn't intended to end up with quite so many people behind bars.

In fact, the bill, which passed with a large, bipartisan majority including liberal stalwarts like Bernie Sanders and Ted Kennedy, wasn't a major cause of mass incarceration in a literal sense. But both the bill itself and, in some ways more importantly, the rhetoric Clinton used about the bill (which was different from the rhetoric Sanders used) were part of the larger trend of "tough on crime" politics that unquestionably drove America's prison population to new heights.

And when trying to understand what really caused mass incarceration, it's important not to overthink it. Mass incarceration happened because mass incarceration was popular.

The crime rate was high in the 1980s and '90s, so there were plenty of criminals to lock up. And people wanted them locked up. The public favored longer, harsher prison terms, more executions, and a punitive rhetoric that would back those things up. Indeed, the public didn't just favor these things — it demanded them. Crime and punishment was a voting issue for the 1990s electorate, and most politicians representing any kind of substantial urban area embraced harsher punishment of criminals.

The exploding prison population wasn't an accident; it was politicians' response to objective political pressure to keep felons in jail longer. The lesson elected officials took from the Willie Horton ad was that if you let a criminal out of jail and he committed another serious offense, it could end your career. So they responded by making it much harder to get out of jail.

Crime was a first-tier political issue in the '90s

The joint effect of the precipitous decline in crime during the late 1990s followed by the 9/11 terrorist attacks led the issue to disappear entirely as a subject of national politics during the aughts. When criminal justice issues resurfaced as a major political issue in recent years, it was progressive activists who put it on the agenda — raising concerns about police treatment of African Americans and about the sheer scale of imprisonment in the United States.

And even as criminal justice reform has worked its way onto the agenda, it's done so as a second-tier issue. An important one that many politicians address — either willingly or at the behest of activists — but not the centerpiece of anyone's campaign.

For people who don't directly remember the political dynamics of Clinton's first term, it can be difficult to imagine how high-profile the crime issue was in the politics of the time.

But as Donald Kaul wrote in a 1994 Philadelphia Inquirer article fact-checking crime-related campaign ads: "There is hardly a campaign for any office that doesn't feature the two candidates wrestling each other into the mud over who is tougher on crime." Kaul wrote that the crime issue "has grown into the 900- pound gorilla of American politics."

Tough on crime was popular in blue states

Lots of things are popular, of course, and that doesn't mean politicians will do them. Raising taxes on the rich polls well, for example, and it's certainly an issue Democrats run on. But Republicans uniformly oppose them, and Republicans representing red states or congressional districts don't worry that the tax issue will prove explosive and sink them.

The crime issue was different. Because the crime problem was centered in urban areas, tough-on-crime politics resonated in blue states.

Rudy Giuliani was elected mayor of New York City in 1993 on a platform whose content was almost entirely focused on crime.

George Pataki was elected governor of New York in 1994 after a campaign that heavily featured his calls to bring back the death penalty and replace the state's appeals court judges with jurists who would be harsher on criminal defendants.

California Republican Pete Wilson cruised to reelection in 1994 by a 55-40 margin after championing a landmark "three strikes and you're out" law, along with harsh measures against illegal immigrants.

Illinois Republican Jim Edgar's successful 1994 reelection campaign made its very first ad about his opponent's desire to abolish the death penalty.

Conversely, as Naomi Murakawa emphasizes in her book The First Civil Right: How Liberals Built Prison America, many successful Democratic politicians from this era ran on their willingness to embrace big government to fight crime even more harshly than Republicans, with more restrictions on gun ownership and higher taxes to hire cops and prison guards. As Clinton put it in his 1992 campaign, Democrats "should no longer feel guilty about protecting the innocent."

Public opinion has changed, but it remains a challenge for reform

Contemporary politics has a different tone. Mainstream Democrats nowadays are inclined to say too many people are in prison rather than too few, some Republicans have embraced reduced incarceration as a means of cutting public spending, the Black Lives Matter movement has focused debate on whether treatment of criminal suspects is too harsh, and objectively lower rates of violent crime have simply reduced the issue's overall salience.

As an excellent 2014 report on public opinion about criminal justice reform for the Opportunity Agenda project found, there has been meaningful change in public opinion since the 1990s.

"People are backing away from harsh enforcement and sentencing policies, such as mandatory sentencing, and appear more interested in allocating tax dollars toward rehabilitation, treatment, and support efforts," they write. "The public is especially supportive of this for people with low level drug convictions."

But the public still remains unwilling to bear any personal risks for change:

Hesitancy around support for some reentry issues indicates a continued social uncertainty of how to fully accept formerly incarcerated people as deserving and equal citizens. Although the public is willing to assist and forgive individuals who make an effort to change, they do not want this process to affect them at all, such as by living near a halfway house or paying more taxes. Americans persistently believe that in the grand scheme of things, people with convictions are not entirely worthy of the same rights as those without a record.

At a time of high crime and persistent public concern about crime, this set of attitudes led fairly inevitably to a politics of longer sentences and mass incarceration. Given falling crime levels and a reduced overall level of public concern, the space has existed for reform in recent years.

But the basic Willie Horton problem still remains. If you champion a policy that lets some convicts out of prison earlier than they would otherwise be released, one of them might kill someone, and that fact could be used against you in an ad, which evidence suggests would be effective. That's why criminal justice policy became so harsh, and that's why it's so hard to make it less so.