GEOFF BENNETT: As we have discussed, President Trump has signed several executive orders making sweeping changes to the immigration system.

Immigration was one of voters' top issues in the presidential election, with many supporting President Trump's message that the current system is broken.

But how did we get here?

Laura Barron-Lopez takes a deep dive into the history of immigration policies and laws that have led to the complex system we have today.

LAURA BARRON-LOPEZ: Since its founding days, the United States had mostly open borders, welcoming immigrants from across the world to work and build lives here.

In the late 1800s, record numbers of migrants from Italy, Greece and Central and Eastern Europe made the journey on newly invented steamships to the United States.

Many, especially in America's now crowded cities, began to question the open door policy.

DAVID LEONHARDT, The New York Times: There was a huge backlash.

There was substantial antisemitism.

There was substantial anti-Catholic discrimination, particularly against Irish immigrants.

And so, over time, this political pressure built up and built up and built up.

LAURA BARRON-LOPEZ: In 1924, Congress passed sweeping legislation restricting immigration.

DAVID LEONHARDT: Congress nearly closes the door, and the part of the door that's open is really just open to Western Europe.

Except for Western Europe, the quotas that existed for individual countries were almost laughably small.

Sometimes, it was, this country could have 100 people a year move to this country from places like Southern Europe.

LAURA BARRON-LOPEZ: The law, which favored mostly white Europeans, would dominate U.S. immigration policy until the 1960s.

JOHN F. KENNEDY, President of the United States: I think it is not a burden, but a privilege to make this really, as it was for them, a new world.

LAURA BARRON-LOPEZ: Building on a promise by President John F. Kennedy and making it part of the fight for civil rights, President Lyndon B. Johnson pushed to repeal the quota system.

He supported a bill that proponents said would not discriminate against people based on their country of origin.

And they stressed it would prioritize skilled workers and family members of people in the U.S., but would not increase the overall number of migrants.

DAVID LEONHARDT: Those promises were absolutely central to selling the bill, because the advocates of the bill understood Americans didn't want a huge increase in immigration.

LAURA BARRON-LOPEZ: Johnson signed it into law at the base of the Statue of Liberty and emphasized that it did not represent radical change.

LYNDON JOHNSON, Former President of the United States: This bill that we will sign today is not a revolutionary bill.

It does not affect the lives of millions.

DAVID LEONHARDT: The law did in fact transform our immigration system and, in particular, led to vastly more immigration.

LAURA BARRON-LOPEZ: Although the legislation contained annual caps on immigration, there was one important exception, family members of naturalized U.S. citizens.

DAVID LEONHARDT: So if I came and then I wanted to bring members of my family over in subsequent years, only I would have counted toward that quota in the first year, not my extended family members.

LAURA BARRON-LOPEZ: Immigration soon soared.

And, over time, family reunification became the largest type of legal migration to the country.

And the foreign-born population shot up from less than 5 percent in 1965 to just over 15 percent in 2023.

MAN: This is the only area in which the American farm labor supply falls short and is supplemented by Mexican citizens.

LAURA BARRON-LOPEZ: Johnson's administration also oversaw the end of a key legal pathway for migrants to work in the U.S.

The Bracero Program, which had been used since World War II to hire millions of temporary agricultural workers from Mexico, expired in 1964.

Those workers continued to cross the southern border, starting a decades-long rise in illegal immigration.

RONALD REAGAN, Former President of the United States: Future generations of Americans will be thankful for our efforts to humanely regain control of our borders.

LAURA BARRON-LOPEZ: In 1986, President Ronald Reagan helped pass sweeping legislation that granted a pathway to citizenship to nearly three million undocumented immigrants in the U.S.

This first modern attempt to crack down on the border failed to deliver and illegal immigration continued to rise through the '90s.

BILL CLINTON, Former President of the United States: We are a nation of immigrants, but we are also a nation of laws.

LAURA BARRON-LOPEZ: President Bill Clinton endorsed proposals by an independent commission to comprehensively reform the immigration system, but these went nowhere.

DAVID LEONHARDT: You saw pretty quickly opposition come from at least two sources.

So, one was what I would describe as the corporate right, who were worried about the idea of a reduction of immigration reducing the work force and potentially meaning that they might have to pay higher wages and then have lower profits.

And then the other opposition was from nonprofits and advocacy groups, particularly Latino and Asian groups, that basically said, we should have more immigration, not less.

And, over time, those groups within the Democratic Party would really become the dominant voice on immigration, at least until the last couple years, of saying more immigration is better.

LAURA BARRON-LOPEZ: Those tensions continued to plague efforts led by presidents on both sides of the aisle to pass comprehensive immigration reform.

THERESA CARDINAL BROWN, Bipartisan Policy Center: What comprehensive immigration reform referred to was essentially three major provisions.

One would have addressed the border and enforcement.

One would have addressed the status of the undocumented in the United States.

And then the third was changes to the legal immigration system.

The concept strategically was that, by putting everything together, there'd be something for every stakeholder group to like.

Unfortunately, it also meant that there were things that every stakeholder group could not like.

LAURA BARRON-LOPEZ: Congress failed to pass bipartisan bills proposed under George W. Bush and Barack Obama.

Under President Obama, deportation soared to more than 400,000 in 2013.

Almost half of those people were deported under a process called expedited removal, which allows immigration officials to deport individuals without a hearing.

JOHN SANDWEG, Former Acting ICE Director: Those individuals were actually apprehended at the border trying to enter the United States.

ICE can remove those people very quickly without having to go through the immigration courts.

It artificially increases the total number of ICE deportations in a way that's a little bit misleading.

Unfortunately, it gave President Obama this kind of name, the deporter in chief.

LAURA BARRON-LOPEZ: By 2016, the situation at the border had transformed.

Migration from Mexico began to drop, while increasing numbers of Central Americans began to surrender to border control and claim asylum.

Under U.S. law, anyone who claims asylum is entitled to a hearing before an immigration judge and cannot be deported.

THERESA CARDINAL BROWN: Many of them really are fleeing dire circumstances, whether that's due to extreme poverty or crime, corruption by their governments.

Many of them do have viable asylum claims.

And we have seen estimates of around 40 percent of the claims that do get through the system are approved.

LAURA BARRON-LOPEZ: Because of immigration court backlogs, this process can take years, and asylees are allowed to live and work in the country until their case is heard.

JOHN SANDWEG: Smugglers seized upon this opportunity to start coaching people to make asylum claims.

Our inability to fund our immigration courts to handle all these claims created a magnet that drew more people here to this country.

LAURA BARRON-LOPEZ: In recent years, this backlog has ballooned from just over 500,000 in 2016 to approximately 3.5 million last year.

DONALD TRUMP, President of the United States: They're bringing drugs, they're bringing crime, they're rapists and some, I assume, are good people.

LAURA BARRON-LOPEZ: An anti-immigrant agenda was central to Donald Trump's first run for president.

As president, he made wide-ranging changes to asylum procedures, including Title 42, which denied entry to asylum seekers under the justification of the COVID-19 pandemic.

JOE BIDEN, Former President of the United States: What we're doing now is attempting to rebuild, rebuild the system that can accommodate what is happening today.

LAURA BARRON-LOPEZ: President Joe Biden reversed many of those Trump era policies and oversaw the largest immigration surge in recent history.

From 2021 to 2023, annual net migration, both legal and illegal, averaged 2.4 million per year.

JOHN SANDWEG: Our failure to enforce our laws at the border, to timely provide due process to people at the border and the perception that the border was out of control, I think that more than anything influenced the thinking of people where they would accept this insane rhetoric we're seeing today.

LAURA BARRON-LOPEZ: On the campaign trail, President Trump stepped up his anti-immigrant rhetoric.

DONALD TRUMP: They're poisoning the blood of our country.

That's what they have done.

DAVID LEONHARDT: So the Republican Party became extremely anti-immigration, sometimes giving voice to false and racist conspiracy theories.

The way that Donald Trump appeals to nativism, the way he appeals to xenophobia, you absolutely can see echoes from that to the 1924 law.

It's absolutely there.

LAURA BARRON-LOPEZ: Trump and his advisers have promised radical changes to the immigration system, from gutting legal pathways like refugee admissions to launching the largest mass deportation program in American history.

President Trump's advisers claim they will prioritize deporting undocumented migrants deemed national security or public safety threats.

But experts say the deportation numbers Trump is talking about would go far beyond that, reaching a scale potentially never seen before in America.

JOHN SANDWEG: There is no way to do the mass deportations they're talking about and focus them exclusively on public safety threats.

The more you push the agency to say, increase the total, that pushes you towards the lowest hanging fruit of the immigration system, which are not the hardened criminals.

When you do a workplace raid, you don't get gang members.

You get people who show up for work every day because they're trying to support their family.

LAURA BARRON-LOPEZ: Laying the foundation for unprecedented action he can now take in office.

For "PBS News Hour," I'm Laura Barron-Lopez.