(CNN) The speed with which the progressive push to "Abolish ICE" moved from activist circles to the national political stage has Democratic leaders scrambling to craft a coherent message, consider their tactics and, in some cases, simply figure out where they stand.

After a weekend that saw massive, nationwide protests against Trump administration border policy, with many in the crowds calling for the elimination of US Immigration and Customs Enforcement, the White House is ratcheting up its defense of the agency, and on Monday it took aim -- with a pair of baiting, bogus online attacks -- at a pair of big-name critics

But while this specific debate is new -- ICE itself is a recent player on the scene, created as part of the federal government's post-9/11 security overhaul -- the conflicts at its core are not, with roots in clashes that go back a century. And while President Donald Trump has escalated ICE's role, immigration advocates are careful to note that his predecessor deserves a share of the responsibility for how the agency is operating today.

CNN spoke to Boston College Law School professor Kari Hong , an immigration expert, attorney and long-standing critic of the agency, about how we got here and what's been missing from the debate.

The conversation has been edited and condensed.

For people who want to understand the history of how we got here, where do they need to start? How far back?

I think it's important to recognize that we in many ways had did have open borders for the first 120 years of our country. As a practical matter, the laws did not kick out anyone until 1917. Before then, Congress had excluded people based on their nationality or their race, but 1917 was a radical shift -- the first time that people could be kicked out after they'd already obtained legal status.

The 1920s, though, are an interesting place to start because there was a very restrictive movement in Congress to actually limit the number of immigrants. There was a congressional bill, supported by the KKK, to limit immigration based on particular nationalities. Italians, for example, were among those excluded at the time because they were perceived as the "bad immigrants" of the day.

Immigration policy was overhauled pretty thoroughly during the civil rights era. What changed and why was it so necessary at the time?

Most scholars agree there were two important changes in the Immigration and Nationality Act of 1965. The first was to abolish this country-based preference for immigration.

The second part is that it really cemented the notion that the American immigration system would be based around family.

Starting at that point, and even today, about two-thirds of all legal immigration comes through family. It is the spouse, the child or the parent who's able to then join their American citizen family member in this country.

How did Reagan's 1986 Immigration Reform and Control Act, which people mostly recall as "amnesty," change the tone of the discussion around immigration?

So yes, on the one hand it did have amnesty. But it also increased immigration enforcement -- both in funding and in resources and attention.

Some scholars have even gone as far to say that this whole term, "illegal immigrant," contributed to increased enforcement. It is not a real term. It doesn't exist in immigration law. But it was used by bureaucrats in the 1970s and 1980s to try and justify getting more money. So that they could have a mission and a purpose.

I think the problem with the term is that it suggests someone has broken our criminal law, when immigration violations are not criminal laws. They're civil matters. That's why people aren't guaranteed a right to a lawyer in immigration hearings. Because it's a civil matter, not a criminal one.

Rather, if you have grounds for relief, such as asylum, you're married to a citizen or you have a family member or work relationship, you get to stay and get a green card. So even though we spend $18 billion each year arresting and detaining immigrants, studies show that if they have a lawyer between 60% and 80% will walk out with green cards. So we're spending money to arrest people that we then give green cards to.

That's very different than arresting criminals who violated the law and will be punished with prison.

After 9/11, the Immigration and Naturalization Service, or INS, was eliminated. As part of that overhaul, the Department of Homeland Security was created and, as part of it, ICE. First things first -- how would you compare ICE's mission to how it is actually employed?

ICE has two missions and this is where we're seeing a big conflict.

It has two separate agencies within it. It has ERO (Enforcement and Removal Operations), which is dealing with deportations -- which deals with arrests and deportations after people have gotten a hearing, if one is due. Then it has HSI (Homeland Security Investigations), which is the agency that investigates actual transnational criminal activity.

What's happening right now is that HSI -- the people who are going after the drug dealers, child pornographers, the sex traffickers -- are saying that ICE is interfering with their mission, that the local police won't cooperate with them and also that ERO has taken up a substantial amount of their money.

And instead of actually having the resources to target real criminals, we've seen ICE spending this money driving around, picking up parents who drop off their kids at school, picking off parents walking on the sidewalk with their children next to them, and in some cases going door-to-door to go after green card holders who are here lawfully.

So it appears that they are more interested in creating a lot of fear and anxiety than actually arresting people whose presence is in any way detrimental to our country's interests.

I think there's another important distinction to make. Between ICE and Customs and Border Protection, or CBP. A lot of people seem to think that both are used to defend the border. How are their missions and responsibilities different?

They're very different. The Border Patrol is in charge of physically being on the border, monitoring who's coming in. The CBP are the people who are at the border making sure that only people and things that are supposed to come in are, in fact, coming in.

There are no mainstream calls to abolish the Border Patrol.

There are growing calls to abolish ICE because it has become so unwieldy with its mission. It is in charge of the interior, but as we've seen, they've really ramped up their attacks on people who are not causing harm.

Something to point out, which I think a lot of people don't understand, is that ICE is not the police. If a bank robbery happens in front of an ICE officer, they have no legal authority to arrest the bank robber -- unless he has committed some immigration violation, or they suspect he has. Like if he's overstayed his immigration visa. And it would be for an immigration violation, not for any crime.

The calls to abolish it also stem from the agency not showing political independence.

Its union endorsed (Trump) during the 2016 election. I can't even imagine if a similar group in the State Department or Environmental Protection Agency went ahead and endorsed someone for president. People would be shocked. And when Obama was in charge, they would openly resist him. (The union) sued him and many agents refused to follow his directive to close cases that weren't warranted for immigration prosecution. Losing political independence hurts the agency's ability to operate credibly.

Speaking of Obama, how much of the current crisis at the border, alongside the concerns and anger over ICE's behavior, is the responsibility of the Bush and Obama administrations?

The major difference from the past is that this is out in the open -- that everyone is actually seeing it.

Immigration advocates were very critical of Obama. But people didn't believe us. He was the "hope and change" guy. How could he be so cruel? But (former Vice President Joe) Biden was sent to Central America, falsely telling the families that their children will not get asylum. Obama has an interview with (ABC News anchor) George Stephanopoulos, saying to people: Don't send your children. We will send them back.

So many of the things are happening have been happening for a long time. And again, they were wrong then and they're wrong now. But the stupidity of one administration cannot justify the malice of another.

Now, because of the heightened level of cruelty, it's visible.

And with the John Kelly memo, when he disbanded the priorities -- often known as the "felon, not families" policy -- and said everyone is subject to deportation, I think people are really realizing that (an arrest-them-all policy is) untenable.

Trump and some other Republicans say people who want to abolish ICE are advocating open borders. Or no borders. Is that accurate?

No. No one (in the mainstream movement) is calling for open borders. This is a false divide. We can, however, have regulated borders, which is what we had before 1996.

What changed in 1996?

The law called IIRIRA (the Illegal Immigration Reform and Immigrant Responsibility Act of 1996, or "Ira-Ira").

Before 1996, if you're in the country and basically were a good person -- if you've been here for seven years, paid your taxes -- you could get a driver's license, get a Social Security card without status. So people integrated into the system, into society and their communities. If you could show the judge that you're a good person, a good neighbor, you were given a green card. And you were allowed to stay and you were expected to continue to contribute, which is what people did.

In 1996, this was cut off. The path to legalization was cut off. And that's why we've got 11 million people in the shadows -- because our courts no longer let people earn a way to stay, and if they left the country they would be barred from returning for three to 10 years.

So people were literally stuck. They couldn't stay and they couldn't go. People often talk about immigration as being a line. No, it's a Rubik's cube and for many people the stickers have been taken off. And it will never, ever be solved because they've been denied a way to legalize. I think that's what people don't realize, and the people who are being denied a way to legalize include veterans, include parents, include beloved community members.

Something I don't think most people get -- yet -- is that "Abolish ICE" is about more than what happens to ICE itself. You've tweeted the hashtag. Why, as someone who has dealt with not just the politics of this, but the reality -- in courtrooms -- of our immigration system, did you take note of it and get behind it?

It resonates with me because what it signals, in a very, very short message, is that it's time to return to common sense.

We cannot be arresting parents. We cannot be arresting and deporting veterans. We cannot be arresting and deporting people for marijuana offenses when so many Americans can now live and buy marijuana legally. Study after study, and economist after economist, shows that our country needs immigrants. Our workforce needs immigrants. Our labor markets use immigrants. Our families are immigrants. So for me this resonates because it's a cry, a call for common sense to come back to immigration policy.