Both sides of the aisle agree that the current US immigration system is broken. It's why immigration's stayed a hot-button political issue and policy debate, and part of what has made Donald Trump the likely 2016 Republican nominee for president.

But the system hasn't always been broken. Or rather, it hasn't always been broken in this particular way.

Everyone remembers that in 1986, President Ronald Reagan passed an "amnesty" law. But what most people don't know is that in 1996 — fresh off the heels of signing welfare reform, and two years after signing the "crime bill" — President Bill Clinton signed a bill that overhauled immigration enforcement in the US and laid the groundwork for the massive deportation machine that exists today.

Both welfare reform and the crime bills Clinton signed have been relitigated during a contentious Democratic primary, but the 1996 immigration bill — the Illegal Immigration Reform and Immigrant Responsibility Act — hasn't.

That's mostly because Democrats have come a long way on the issue since 1996, and advocates have been happy to let them do it without asking too many questions about the past. Only now are some progressive Democrats trying to raise the issue (32 members of the House of Representatives have signed onto a congressional resolution condemning the 1996 law, introduced Thursday by Rep. Raul Grijalva).

If Democrats ever find themselves in a position to pass the comprehensive immigration reform, they might find the past law's immigration legacy has been too consequential to ignore.

What '90s immigration reform did: made more people deportable and fewer people legalizable

There was no single provision of the 1996 law that was as dramatic as the 1986 "amnesty" law, signed by President Reagan, which is why he gets credit for the last major immigration reform. But the '96 law essentially invented immigration enforcement as we know it today — where deportation is a constant and plausible threat to millions of immigrants.

It was a bundle of provisions with a single goal: to increase penalties on immigrants who had violated US law in some way (whether they were unauthorized immigrants who'd violated immigration law or legal immigrants who'd committed other crimes).

Most immigration wonks call the 1996 law IIRIRA (pronounced "Ira-Ira") — and it's far from beloved by them. Here are some of their most significant complaints:

More people became eligible for deportation. Legal immigrants — including green-card holders — can be deported if they're convicted of certain crimes (which cover a broad umbrella of offenses, some of which aren't violent). But in 1996, Congress radically expanded which crimes made an immigrant eligible for deportation. And they made these changes retroactive.

"Overnight," says law professor Nancy Moravetz of NYU, "people who had formed their lives here — came here legally or had adjusted to legal status, were working here, building their families, had ordinary lives in which they were on the PTA and everything else — suddenly, because of some conviction, weren't even allowed to go in front of a judge anymore. They were just fast-tracked to deportation."

It got easier to deport people. Immigrants convicted of crimes weren't the only ones stripped of the ability to argue their case before a judge before getting deported. So did anyone apprehended within 100 miles of the border. And IIRIRA required the government to hold more immigrants in detention before deporting them — making it substantially harder for them to get lawyers.

These changes drastically reduced the amount of leeway that immigration judges and the executive branch had to exercise discretion in whether or not to deport an immigrant.

"Discretion was taken away from district directors and immigration judges almost entirely," says Doris Meissner, who was head of the Immigration and Naturalization Service at the time. "And so deportations started to go up, people were deported who otherwise would not have been deported."

The change to the law was so drastic that after a high-profile deportation of an immigrant over a minor crime led to public outcry, Republican members of Congress — including the lead author of IIRIRA — wrote the Clinton administration asking them to back down.

It got a lot harder for unauthorized immigrants to "get legal." For much of the 20th century, it was possible for at least some unauthorized immigrants to obtain legal status once they'd been in the US for a certain amount of time. Before 1996, for example, immigrants who'd been in the US for at least seven years could get legal status as long as they showed it would cause them "extreme hardship" to get deported.

These standards weren't easy to meet. But IIRIRA made them essentially impossible.

It limited "cancellation of removal" to immigrants who'd been in the US for at least 10 years. Instead of having to show that the immigrant herself would suffer "extreme hardship" if she was deported, she'd have to show that a US citizen (like her spouse or child) would suffer "exceptional and extremely unusual hardship." The simple fact that the family would be separated if she were deported wouldn't count. And the US could only grant this to 3,000 immigrants each year.

That essentially eliminated an existing back door to legal status. But IIRIRA did even more. It locked a front door to legal status, too.

Marrying a US citizen or permanent resident makes you eligible to apply for a green card. So does having an immediate relative who's a US citizen (like a child), as long as the citizen's over 18. These are true whether or not you already live in the US. And before IIRIRA, it was true regardless of whether or not you were legal to begin with.

Starting after IIRIRA passed in 1996, though, an unauthorized immigrant couldn't directly apply for legal status — even if he had married a US citizen, or qualified for a green card through a relative. Immigrants were banished for at least three years if they'd lived in the US without papers for six months; the banishment lasted 10 years if the immigrant had lived in the US without papers for a year or more.

You could waive these bars if you could show that your spouse or child would suffer "extreme hardship" — but you had to leave the country to do it, triggering the ban before you found out if you'd gotten the waiver. Many immigrants understandably felt it wasn't worth the risk.

The provision became known as the "3- and 10-year bars" — a technical-sounding term that is so widely known and reviled among immigrants that Hillary Clinton uses it in stump speeches.

This law laid the framework for modern spikes in deportation

"I don't think people fully appreciated what those laws had done," says Nancy Morawetz, referring to both IIRIRA and the other 1996 laws that affected immigration. In some ways, they're "still being sorted out today."

But one effect was clear: After IIRIRA, deportation from the United States went from a rare phenomenon to a relatively common one. "Before 1996, internal enforcement activities had not played a very significant role in immigration enforcement," sociologists Douglas Massey and Karen Pren have written. "Afterward, these activities rose to levels not seen since the deportation campaigns of the Great Depression."

This particular law was passed during an era where Congress and the Clinton administration were both working to increase the amount of spending and agents on the US–Mexico border.

And after 9/11, the way the federal government handled immigration changed in two major ways. The bureaucracy was reorganized — and moved from the Department of Justice to the Department of Homeland Security. And the funding for immigration enforcement got put on steroids.

The combination of those gave rise to what Meissner and the Migration Policy Institute have called a "formidable machinery" for immigrant deportations — a machinery that took the US from deporting 70,000 immigrants in 1996 to 400,000 a year though the first term of the Obama administration. But that machine was built on the legal scaffolding of the options IIRIRA opened up.

"Both of those things have had so much more force because of this underlying statutory framework that they were able to tap into," says Meissner. In retrospect, "it was sort of a perfect storm."

After '90s immigration reform, the unauthorized population tripled

But even though deportations exploded after the passage of IIRIRA, it didn't keep the population of unauthorized immigrants in the US from growing. It went from 5 million the year IIRIRA was passed to 12 million by 2006. (By contrast, during the decade between the Reagan "amnesty" and IIRIRA, the unauthorized population grew by only 2 million.)

These two things didn't happen despite each other. More immigration enforcement is one big reason why there are so many unauthorized immigrants in the US today.

A lot of this is because of the increase of enforcement on the US–Mexico border — something that was happening even without IIRIRA. Many unauthorized immigrants used to shuttle back and forth between jobs in the US and families in Mexico. Once it got harder to cross the border without being caught, they settled in the US — "essentially hunkering down and staying once they had successfully run the gauntlet at the border," as Massey and Pren write — and encouraged their families to settle alongside them.

(This wasn't the only reason unauthorized immigrants started settling in the US around this time. The types of jobs available for unauthorized workers were changing, with seasonal agricultural jobs being replaced by year-round service-industry ones, for one thing. But it was certainly a major factor.)

But if border enforcement encouraged families to stay, IIRIRA prevented them from obtaining legal status. By this point, a majority of the unauthorized-immigrant population of the US has been here 10 years — more than enough time to qualify for cancellation of removal, if IIRIRA hadn't made it so difficult to get. Millions of them have children who are US citizens.

The 3- and 10-year bars alone have caused millions of immigrants to remain unauthorized who'd otherwise be eligible for green cards or US citizenship by now. According to Douglas Massey's estimate, if those bars hadn't been instituted in 1996, there would be 5.3 million fewer unauthorized immigrants in the US today. In other words, the population of unauthorized immigrants in the US would literally be half the size it is now.

A Republican bill that Democrats couldn't vote against

So who's to blame for all of this?

Unlike some of the Clinton-era laws that the Democratic Party has now moved to the left of — like the 1994 crime bill and welfare reform — IIRIRA was not President Clinton's bill. It was Republicans who'd pressed the issue of tightening immigration restrictions during the 1994 campaign (both in Congress and in California, where Gov. Pete Wilson rode to reelection on a ballot proposition severely restricting unauthorized immigrants' use of state services like public schools).

When Republicans won the House of Representatives in 1994, they — and especially Rep. Lamar Smith (R-TX), the new chair of the Immigration Subcommittee of the House Judiciary Committee — came in with a mission. "They were about the business of really toughening up immigration law," says Doris Meissner, who was head of the Immigration and Naturalization Service at the time. "And that is what they did" — sticking immigration provisions in welfare reform and the Anti-Terrorism and Effective Death Penalty Act of 1996 (or AEDPA).

And then there was IIRIRA, which was originally introduced as a comprehensive immigration enforcement bill: seriously tightening the requirements for legal immigration; making it harder to apply for and receive asylum in the US; and increasing immigration enforcement.

"Nobody really felt like they had a lot of leverage" against the Republican plan, says Charles Kamasaki of the National Council of La Raza.

Pro-immigration Republicans and Democrats were able to limit the damage by dividing the bill. They blocked the restrictions on future legal immigration, and were "at least partially successful in mitigating" restrictions on asylum (in Kamasaki's telling).

But at the heart of the split-the-bill strategy was the recognition that the enforcement provisions against "criminal aliens" were too popular to stop — not only among Republicans, but among congressional Democrats and the Clinton White House.

"There was a pretty spirited fight on the 3- and 10-year bars" in Congress, says Kamasaki, as well as on a few other amendments. "But the votes weren't even close."

The administration certainly didn't seem to have a problem with the enforcement provisions of IIRIRA. "We all understand the problem of illegal immigrants. We're all trying to ensure that we have additional enforcement to protect against illegal immigrants," said White House Chief of Staff Leon Panetta at the time. "But I, for the life of me, do not understand why we need to penalize legal immigrants in that process."

Privately, Meissner says now, "There were many parts of it that required really swallowing hard."

But publicly, the White House was enthusiastic — and reinforced the idea that while restrictions on legal immigrants and immigration might be controversial, getting tough on immigrants who'd violated the laws was not.

In a press conference after President Clinton signed IIRIRA into law, Panetta crowed: "We were able, I think, as a result of this negotiation to be able to modify — eliminate — the large hits with regards to legal immigrants, while keeping some very strong enforcement measures with regards to illegal immigration."

The Clinton White House wanted an "opportunity" to demonstrate it was tough on immigrants

If IIRIRA was as terrible a bill as Meissner claims, why did Panetta celebrate signing it? For that matter, why did President Clinton sign the bill at all?

The answer is, essentially, that on some level the Clinton administration really did want to look tough on immigration. And that was more important than vetoing a bill because some in the administration didn't like its policy provisions.

"It's certainly the case that the administration was enforcement-minded where illegal immigration was concerned," Meissner says. That started at the top.

Bill Clinton had personal experience with immigration as a political liability: the only election loss of his career (his gubernatorial reelection campaign of 1980) came after he'd agreed to house Cuban refugees in Arkansas after the Mariel boatlift. He was convinced, even as president, that being soft on immigration was a no-go for Democrats — just like being soft on crime or welfare.

So from one angle, the administration painted itself into a corner with IIRIRA: It had to sign any bill Congress offered, and this was the one it got.

"The administration was taking a position that immigration enforcement needed to be strengthened," says Meissner. "Under those circumstances, you've got to try to get as good a bill as you can get. But if you veto a bill — it would have been viewed as politically dishonest."

But the Clinton administration might not have been as reluctant to sign IIRIRA as Meissner implies.

In a memo written in November 1996, a few months after IIRIRA was passed, a senior adviser to the president named Rahm Emanuel wrote a memo recommending a series of aggressive steps President Clinton could take in the wake of the law — including "claim and achieve record deportations of criminal aliens."

"After the Crime Bill passed in 1994, we built a strong record on crime," Emanuel wrote. "The illegal immigration legislation provides that same opportunity; now that the legislation is passed, we can build up a strong Administration record on immigration."

Democrats swiftly moved left on immigration since then — and most advocates are happy to leave the past in the past

Despite Emanuel's prediction, though, immigration and crime have followed totally different trajectories for the Democratic Party over the past two decades. While criminal-justice reform has only recently become a consensus issue among Democrats — and many of them are still less enthusiastic than certain reform-minded Republicans — comprehensive immigration reform, including a path to citizenship for the 11 million unauthorized immigrants currently in the US, has enjoyed unanimous support among Democrats for nearly a decade.

The shift started in the years right after IIRIRA's passage. In 1997, Congress passed a law protecting some Central American asylum-seekers from deportation. In 2000, it passed a law making it a little easier for people to immigrate legally to the US to be with relatives. By 2000, Charles Kamasaki says, with the exception of "two or three" Democrats in each chamber, "it was pretty clear" that the Democratic Party stood with immigration advocates.

Advocates, for their part, welcomed Democrats with open arms. When Democrats who'd previously been "enforcement-minded" on immigration started emphasizing the need to let unauthorized immigrants get citizenship — up to and including Rahm Emanuel, who as mayor of Chicago has been a loud supporter of "welcoming" immigrants — many advocates praised them for "leaning in" on the issue. The harsh words of the past, or the signing of bills like IIRIRA, were only mentioned to point out how much the Emanuel wing of the party had evolved.

This approach had its advantages: It helped immigration reform become a Democratic priority, rather than one that split both major parties. But it also meant there was no opportunity to reckon with the effects of the 1996 law, because no one had an incentive to bring them up.

Immigration-enforcer Republicans could use the 1986 "amnesty" against their colleagues, in a tone of "We tried this once, let's never try it again." But immigration-reformer Democrats didn't have any reason to remind the public that any Democrat had tried enforcement at all.

This isn't to say that none of IIRIRA's provisions have come under criticism. In particular, Democrats have started turning against the 3- and 10-year bars — the IIRIRA provision that's done the most to keep unauthorized immigrants from getting legal.

President Obama made it easier for some immigrants to apply for waivers from the bars without leaving the country. Hillary Clinton has promised to pass a law getting rid of them entirely. But as Bernie Sanders — or rather, Bernie Sanders's campaign Twitter account — pointed out when Clinton made this promise at a debate, she neglected to mention her husband had signed the bars into law.

The legacy of "felons, not families"

The 3- and 10-year bars might be the single biggest issue with IIRIRA, but they're hardly the only things keeping immigrants from becoming legal, or dooming them to deportation. "We haven't seen anybody speak out about the issue of limited discretion and over-enforcement for people who have any kind of criminal issue," points out Nancy Morawetz. "And that's a problem."

Indeed, even the current, more progressive Democratic message on immigration reinforces one of the biggest themes of IIRIRA, and the deportation regime it laid the groundwork for: that immigrants with criminal involvement ought to be deported, with no questions asked.

President Obama loves to say that he's trying to deport "felons, not families." But as Morawetz says, that rhetoric "ignores the fact that people who might have a felony conviction 20 years ago have families." Should they be deported? IIRIRA says yes. No Democrat has yet been able to say no.

This continued desire to stay tough on "criminal aliens" has made it harder for Democratic administrations to restrain enforcement — even to bring it back to Clinton-era levels, when the rhetoric against "illegal immigration" was harsher than it is today. "Criminal aliens" were one of the chief drivers of the record-setting deportation rates of Obama's first term.

After IIRIRA passed, Doris Meissner's INS managed to stall a program that would have allowed local law enforcement to enforce federal immigration laws. When cities and counties started asking to get approved for the program, she says, INS "said that we wanted to participate in an across-the-board community discussion" about how they'd use their authority before signing a memorandum of agreement. "After going through maybe three or four of those in jurisdictions, it became clear how complicated it was," and interest disappeared.

The program was reanimated and given new teeth under the Bush administration, however. And under Obama, local/federal cooperation on immigration law has become the rule — even if local police officers themselves aren't always involved. Indeed, when President Obama attempted to reform his signature local/federal cooperation program by including, among other things, the input of local stakeholders — exactly what Meissner had done in the late 1990s — it was treated as politically controversial.

In other words, the '90s reform shaped the very framework with which we're using to discuss immigration reform today.

Will Democrats' inability to reckon with their past limit the effectiveness of immigration reform?

Right now, this is somewhat of an academic conversation: with Republicans controlling Congress and Democrats controlling the White House. But if Democrats manage to retake Congress in 2016 and keep the White House, they may find themselves with a real shot at passing comprehensive immigration reform.

One of the biggest sticking points with comprehensive immigration reform is that everyone wants to impose certain requirements on who can qualify for a path to citizenship — but because the unauthorized population is in the shadows, no one knows exactly how many people would qualify for reform under a given set of requirements.

In 2013, the Congressional Budget Office estimated that only 8 million of the 11 million unauthorized immigrants in the US would end up becoming citizens under the requirements of the Senate's immigration bill, which briefly looked like it might actually happen. But it didn't explain how it arrived at that number, or how many people it thought would be excluded based on the bill's various requirements for legalization — one of which excluded most people with criminal records.

"Because of the lack of measurable standards to estimate the affected population," says Jose Magana-Salgado, managing policy attorney for the Immigrant Legal Resource Center, "it is likely that criminal bars will inadvertently exclude a larger than expected number of people from relief under immigration reform. And because we'd only find out the breadth of this exclusion after the passage of reform — a once in a lifetime event — those people would remain forever excluded from permanent status and mired in the shadows."

Of course, it's very difficult to get politicians to care about something whose effects can't be measured. That's one of the bitter lessons of the '96 law: If the consequences of a law are indirect enough, it's very easy for people to forget that it's there.