AMNA NAWAZ: Well, there was an unprecedented scene at the U.S.

Supreme Court today.

Donald Trump became the first sitting U.S.

president to attend oral arguments in a case that will decide the fate of one of the biggest and most controversial policies of his second term.

Our justice correspondent, Ali Rogin, has more on the consequential legal debate before the justices.

ALI ROGIN: On his first day back in office last year, President Trump signed an executive order seeking to end birthright citizenship, a cornerstone of immigration policy enshrined in the 14th Amendment to the Constitution and affirmed by the Supreme Court more than 100 years ago.

The principle is simple.

If you are born here, you are a citizen.

But now the justices are reexamining the core questions.

What makes an American citizen, and who gets to make the rules?

For a closer look at today's arguments and legal questions, we're joined now by our Supreme Court analyst and SCOTUSblog co-founder Amy Howe and Amanda Frost, professor of immigration and citizenship law at the University of Virginia.

Thank you both so much for being here.

You were both at the court this morning, and so was President Trump.

Amy, I want to start with you.

What was it like in the courtroom with him there?

AMY HOWE: So there's always a lot of security, and it's always a pretty solemn ceremony.

But there was sort of an extra layer of security, because, on top of the Supreme Court police, we also had a lot of Secret Service agents, extra police cars parked outside.

In terms of the president's presence himself, it really didn't make that much of a difference.

He sort of slipped in about 13 minutes before the argument started, and then he slipped out again shortly after his solicitor general, John Sauer, finished his time at the lectern.

There wasn't sort of an announcement that the president is here or that the president is leaving.

But the solicitor general himself, John Sauer, was very animated at the oral argument today.

And so whether or not that was a response to the fact that his boss was in the room watching, we won't know, but it certainly seemed possible.

ALI ROGIN: Did the justices have any response to President Trump being there that you could see?

AMY HOWE: None at all.

He was not acknowledged.

ALI ROGIN: Amanda, the administration sought to sort of give their argument as for what citizenship should really be defined as.

So what were they arguing makes a citizen, and how does it comport with these 100 years of legal precedent?

AMANDA FROST, University of Virginia School of Law: Yes, so this began on the first day of President Trump's presidency when he issued an executive order in which he said that birthright citizenship, which is guaranteed in our Constitution, he said it only applies to the children of U.S.

citizens and children of green card holders.

So that would exclude all the children of undocumented immigrants and also all the children born to temporary visitors, which includes people like H-1B skilled workers, student visa holders, as well as tourists.

So those are the arguments that we saw Solicitor General Sauer make today to try to support that executive order.

ALI ROGIN: And what was he trying to justify that on?

Is there -- was there language based in the 14th Amendment to the Constitution that he was saying indicates that position?

AMANDA FROST: So, yes, it all turns - - there's this one sentence in the 14th Amendment that said all persons born are naturalized in the United States and subject to the jurisdiction thereof are citizens of the United States.

So it all turned on that language subject to the jurisdiction thereof.

And the administration tried to say that's -- that language carves out all the children born to those two groups, the undocumented immigrants and the temporary immigrants.

ALI ROGIN: Amy, I want to ask you, what stood out to you in terms of what the justices were asking?

And did any of them seem particularly skeptical about the argument the administration was trying to make?

AMY HOWE: So, one of the things that really stood out was how much time the justices spent discussing an 1898 case called Wong Kim Ark.

So the 14th Amendment was intended to make clear that formerly enslaved people and their children were in fact U.S.

citizens.

But the Supreme Court in this Wong Kim Ark case held not only that it protects formerly enslaved people, but that essentially everyone, subject to a few exceptions, is a U.S.

citizen.

And so both sides in the case are relying on this 1898 decision.

The challengers in the case are saying, look, there's this 1898 case that says everyone is a U.S.

citizen.

The U.S.

government is essentially saying that case was different because the plaintiff in that case, who was born in the United States to the parents of -- who were Chinese nationals, went overseas, tried to come back to the United States.

Customs official said he's not a U.S.

citizen.

The Supreme Court agreed that he was.

But his parents had lived in the United States for a long time.

They were domiciled here.

And that's different from these temporary workers and people who are undocumented.

In terms of some of the conservative justices who were skeptical, Justice Neil Gorsuch was skeptical.

He said the 14th Amendment was enacted when we didn't have strict immigration laws.

There really wasn't such a thing as undocumented immigrants.

And if we looked at whether or not people were domiciled, perhaps a lot of people who are currently undocumented immigrants might in fact pass your test.

Justice Brett Kavanaugh also seemed skeptical of the government's position.

One of the government's arguments is that the United States is one of relatively few developed countries that has birthright citizenship.

And he, in essence, said, why should that matter with -- in terms of how we interpret the Constitution?

We should interpret the Constitution using U.S.

laws, U.S.

history.

That may be useful as a policy matter, but not how we should interpret the Constitution.

ALI ROGIN: Amanda, as somebody who looks very closely at the history of birthright citizenship, what were your main takeaways from the justices' questions today?

AMANDA FROST: Yes, a couple different points.

One is, Justice Barrett, so more often on the conservative side, she had some really hard questions for the solicitor general.

The solicitor general had argued that the children of undocumented immigrants are excluded, and he said they couldn't have intended to include the children of undocumented immigrants back in 1868 because no such category of people existed.

But, in fact, as Justice Barrett pointed out, there had been illegally imported slaves, people that were imported in violation of U.S.

law and enslaved in the United States.

And she said, of course, we all agree they're citizens, and the solicitor general agrees the children of those enslaved people are citizens.

And aren't they just in the same position?

They were illegally present.

They certainly didn't want to stay in the United States.

And she made that point as well.

One other point worth mentioning -- and this was both Justice Barrett and Justice Gorsuch -- it would be very, very difficult to implement the lines the government was trying to draw.

Back in 1868, we didn't have these immigration categories, so how would we determine whether someone was domiciled or not?

And that also goes to the original understanding and undermines the government's case.

ALI ROGIN: Amy, the administration's solicitor general, John Sauer, he pointed to how immigration has changed a lot since the 14th Amendment was ratified.

One of the examples he gave was that of birthright tourism, the practice of coming to the United States in order to give birth.

And that sparked a debate with Chief Justice John Roberts.

Take a listen.

JOHN ROBERTS, Chief Justice of the U.S.

Supreme Court: It certainly wasn't a problem in the 19th century.

D. JOHN SAUER, U.S.

Solicitor General: No, but, of course, we're in a new world now, as Justice Alito pointed out, to where eight billion people are one plane right away from having a child who is a U.S.

citizen.

JOHN ROBERTS: Well, it's a new world.

It's the same Constitution.

ALI ROGIN: Definitely going to be a memorable line there, "new world, same Constitution."

But this administration has made a practice of making these arguments that are sometimes very sweeping and dramatic and point to national survival, which is along the lines of what General Sauer was doing, but did any of the justices seem particularly compelled by that line of argument?

AMY HOWE: No, that was really the only time that birth tourism came up in the oral argument, and it sort of hearkened to me back to the tariffs argument, when the Trump administration had made claims about how important the tariffs were to the economy, and there was the famous line in the government's brief that, without tariffs, we would be a dead country.

And I think that the Supreme Court looks at some of these claims and says, well, that -- again, that may be true as a policy matter.

It may be sort of an interesting data point, but we're not going to let that factor into how we interpret the law in the tariffs case or the Constitution in this case.

ALI ROGIN: Amanda, several justices asked about the real-world consequences of changing birthright citizenship.

Here is what Justice Ketanji Brown Jackson asked about it.

KETANJI BROWN JACKSON, U.S.

Supreme Court Associate Justice: How does this work?

Are you suggesting that, when a baby is born, people have to have documents, present documents?

Is this happening in the delivery room?

How are we determining when or whether a newborn child is a citizen of the United States under your rule?

ALI ROGIN: If the court rules in the administration's favor, how transformative could that be?

AMANDA FROST: So the consequences of this executive order going into effect would be enormous.

So, first, it would prevent about a quarter-million children each year from gaining citizenship going forward, which they would otherwise have under the previous understanding of the citizenship clause.

It also would mean that every family who gives birth to a child going forward, and that's 3.5 million families a year, would have to prove their ancestry, their lineage before their child would be recognized as a citizen.

And, finally, I think it would change the meaning of what it is to be an American.

Born in the USA resonates with people, because that's the definition of American.

And if this executive order went into effect, it would change that for the nation.

ALI ROGIN: Amy Howe, Amanda Frost, thank you so much, both of you, for your insights.

AMANDA FROST: Thank you.

AMY HOWE: Thank you.