We know what we want Judge Jesse Furman to do at today’s hearing on the future of Hamas sympathizer and American green card holder Mahmoud Khalil. It’s to apply the law. In applying the law, the judge could find it helpful to recall a guide to parsing the parchment: “The Constitution is not a suicide pact.” That comes from no less an authority than Justice Robert Jackson, who knew a thing or two about America’s enemies.

Mr. Khalil, a leader of the encampment movement that made Columbia a perilous place for Jews and Israelis, is squarely in that category. Writing in the Sun, Alan Dershowitz avers that “no one should have sympathy for Mr. Khalil as an individual, as an advocate, or as an ideologue. His views, as he himself has expressed them, are despicable, anti-American, antisemitic, and intolerant of others.” Mr. Khalil has called for the ​​“end of Western civilization.” 

Mr. Khalil was arrested by federal immigration agents on Saturday. The White House explains that Secretary Rubio has been “presented with intelligence” that Mr. Khalil, who was graduated in December, is a national security threat. Mr. Trump calls it “the first arrest of many to come.” Fox News reports that the government plans to argue that Mr. Khalil is deportable under the Immigration and Naturalization Act. 

That statute renders deportable an “alien whose presence or activities in the United States the Secretary of State has reasonable grounds to believe would have potentially serious adverse foreign policy consequences for the United States.” It also mandates that any alien who “endorses or espouses terrorist activity or persuades others to endorse or espouse terrorist activity or support a terrorist organization” is ineligible for a visa or entry to America.

The government could have Supreme Court precedent on its side. In a 1904 case, Turner v. Williams, the high court held “that the deportation of an alien who is found to be here in violation of law is not a deprivation of liberty without due process of law.” Mr. Khalil, though, has not been charged with a crime, and he is here legally. He was, though, present at the rampage through Barnard last week. The administration accuses him of being “aligned with Hamas.” 

Turner also held — in respect of anarchists, a source of tsuris in the Gilded Age  — that Congress could legislate against ideologies “so dangerous to the public weal that aliens who hold and advocate them would be undesirable additions to our population, whether permanently or temporarily, whether many or few.” Mr. Trump declares that “pro-terrorist, anti-Semitic, anti-American activity” will not be tolerated, let alone permitted to flourish on campuses.

Enter Jackson’s quip about the suicide pact. He was preceded in the insight by President Jefferson, who with respect to the Louisiana Purchase ruminated that “a strict observance of the written law is doubtless one of the high duties of a good citizen, but it is not the highest. The laws of necessity, of self-preservation, of saving our country when in danger, are of higher obligation.” President Lincoln invoked a similar principle in respect of habeas corpus.

Jackson warned of turning the Constitution into a suicide pact in the case of Terminiello v. Chicago, which concerned the conviction of a priest whose rhetoric precipitated a riot. The Nine overturned his guilty verdict on account of the First Amendment. Jackson warned in his dissent that “if the court does not temper its doctrinaire logic with a little practical wisdom, it will convert the constitutional Bill of Rights into a suicide pact.”

Which brings us back to Judge Furman. What in the world, he could ask, could be more un-American than the kind of hatred of which Mr. Khalil has become a spokesman? He could find that Secretary Rubio is well within his granted powers. He could find that using the First Amendment to protect allies of Hamas in their campaign against Israel and America would turn the Constitution into the very suicide pact against which our sages warned.