The Trump administration continues to claim he violated an executive order prohibiting anti-Semitism, though no evidence was provided. Protesters in NYC demand his release, calling the arrest unconstitutional.

His location remains unclear. The ACLU and immigrant rights groups argue the detention violates free speech, warning it sets a dangerous precedent.

It looks like this is probably an open question in the Constitution. The Supreme Court has, in the past, avoided ruling on the matter.

My understanding is that US border control has generally had pretty broad leeway in terms of disallowing people who are not US citizens into the US. There hasn’t been a Supreme Court case that has stated that First Amendment protections mean that a non-citizens’ speech can be used as grounds for entry or presence in the US.

https://www.freedomforum.org/non-citizens-protected-first-amendment/

Can the government selectively enforce immigration laws based on political views? (1999)

The federal government sought to deport eight people who were members of a U.S.-based Palestinian liberation group. They were legal U.S. residents but not full citizens. The group claimed they were being targeted with selective enforcement because of their political views and appealed to the Supreme Court (Reno v. American Arab Anti-Discrimination Committee). When challenged, the government backed off the political grounds for deportation but proceeded on technical violations of immigration law. In his majority opinion, Justice Antonin Scalia addressed claims of First Amendment violations, saying, “An alien unlawfully in this country has no constitutional right to assert selective enforcement as a defense against his deportation.”

The US Executive Branch effectively prohibits naturalization to Communists, despite the fact that there is First Amendment protection for an American citizen who wants to advocate for such. The way this works is that they ask someone if they’ve been part of a Communist Party. If so, they can prohibit naturalization. If the answer is “no” — and not true — then naturalization can later be revoked as having been obtained on fraudulent grounds.

https://www.uscis.gov/policy-manual/volume-8-part-f-chapter-3

Chapter 3 - Immigrant Membership in Totalitarian Party

A. Purpose and Background

1. Purpose

The inadmissibility ground for immigrant membership in or affiliation with the Communist or any other totalitarian party is part of a broader set of laws passed by Congress to address threats to the safety and security of the United States. Its original purpose was to protect the United States against un-American and subversive activities that were considered threats to national security.

In general, any immigrant who is or has been a member of or affiliated with the Communist or any other totalitarian party (or subdivision or affiliate), domestic or foreign, is inadmissible.[1] There are two exceptions to this ground of inadmissibility and a limited waiver available to certain aliens depending on the immigration benefit they are seeking.[2]

https://en.wikipedia.org/wiki/Yates_v._United_States

Yates v. United States, 354 U.S. 298 (1957), was a case decided by the Supreme Court of the United States[1] that held that the First Amendment protected radical and reactionary speech, unless it posed a “clear and present danger”.

SCOTUS has ruled that the Executive Branch may not constitutionally prohibit a citizen who is a member of a Communist party from traveling abroad:

https://en.wikipedia.org/wiki/Aptheker_v._Secretary_of_State

Aptheker v. Secretary of State, 378 U.S. 500 (1964), was a landmark decision of the US Supreme Court on the right to travel and passport restrictions as they relate to Fifth Amendment due process rights and First Amendment free speech, freedom of assembly and freedom of association rights. It is the first case in which the US Supreme Court considered the constitutionality of personal restrictions on the right to travel abroad.

In Aptheker, the petitioner challenged Section 6 of the Subversive Activities Control Act of 1950, which made it a crime for any member of a Communist organization to attempt to use or obtain a passport.[1]

But the question of whether the First Amendment protection applies to speech used as a criteria for non-citizen entry to the US apparently hasn’t really been resolved:

https://www.nyclu.org/commentary/column-terrorism-international-border-and-first-amendment-new-york-law-journal

Finally, there is Kleindienst v. Mandel, the Supreme Court’s most recent examination of the First Amendment as it applies to the border. Decided in 1972, that case arose out of the government’s refusal to grant a visa to Ernest Mandel, a Belgian scholar who described himself as “a revolutionary Marxist” and who had been invited to speak at various prestigious academic events in the United States. Federal law at that time barred entry into the country of aliens who advocated or published “the doctrines of world communism or the establishment in the United States of a totalitarian dictatorship,” but the statute provided that the government could waive the bar. When the government refused to grant Mandel a waiver, various academics filed suit, challenging the statutory provision as violating the rights of the academics as American citizens to receive information under the First Amendment.

The Supreme Court rejected the challenge and in doing so emphasized Congress’ virtually plenary power over the entry of aliens into the country. Nonetheless, it refused the government’s invitation to hold that that authority trumped the First Amendment in all circumstances involving aliens. Rather, it ruled more narrowly, finding that the government’s refusal to grant Mandel a waiver was based on factors other than his political beliefs, which was sufficient to defeat the First Amendment challenge. Left for another day was resolution of the First Amendment’s reach to the border in disputes over the entry of non-citizens.