President Donald Trump, in his zeal to continue his crackdown on immigrants, has resurrected an old, mostly fringe argument: that the United States should no longer guarantee citizenship to those born on American soil. It is a notion rooted in fear and antithetical to the very Constitution he swore to uphold.

The 14th Amendment is an eloquent, uplifting statement on what it means to be an American. It offers equal protection under the law not just to citizens but to all who reside here. It clarifies — in ways the original Constitution did not — who can be an American citizen: “All persons born or naturalized in the United States and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.”

For 150 years, that has been the constitutional promise of this country, that those lucky enough to be born here would be Americans from their first breath. The notion arose out of the searing divisions left by the Civil War, when some states were determined to withhold citizenship from the children of slaves. Those 1868 authors could have crafted an amendment that extended citizenship to slaves alone. Certainly such arguments were advanced at the time. Instead, the authors went much further.

Anthony Winer, a constitutional law professor at Mitchell Hamline School of Law, calls the 14th Amendment “one of the most consequential and transformative amendments we have.” According to Winer and the vast majority of legal scholars, the clause that Trump and some others are pinning their hopes on, “subject to the jurisdiction thereof,” is widely understood as applying to the children of foreign diplomats and enemy soldiers — not immigrants, regardless of legal status.

Trump may be dismayed that James C. Ho, whom he recently elevated to the vaunted Fifth Circuit Court of Appeals in D.C., is among those legal experts. A conservative constitutional scholar who clerked for Supreme Court Justice Clarence Thomas, Ho offered an eloquent and detailed defense of the amendment in 2011, saying that “the text of the Citizenship Clause plainly guarantees birthright citizenship to the U.S.-born children of all persons subject to U.S. sovereign authority and laws.” Ho said that “the sweeping language” of the 1898 high court decision in U.S. vs. Wong Kim Ark, which turned on this issue of citizenship, “reaches all aliens regardless of immigration status.”

Winer said the understanding of that clause is so widely held that he is dubious that any attorney or expert of standing would tell Trump that he could alter the Constitution by executive order alone. In any reasonable administration, the attorney general would advise the president how unlikely he would be to prevail. A Congress that stood for something besides fealty to the president would defend the Constitution. At least outgoing House Speaker Paul Ryan came out against the proposal, saying, “You cannot end birthright citizenship with an executive order … . We didn’t like it when [President] Obama tried changing immigration laws via executive action and obviously as conservatives we believe in the Constitution.”

Sadly, these are not normal times. Americans can hope — but they should not trust — that these institutions would contain the president’s worst policy instincts. Ryan is on his way out. In the Senate, Republican Lindsay Graham of South Carolina is eager to carry a birthright bill for Trump.

It’s possible that Trump’s plan is simply a last-ditch election ploy to rile his base. But Trump has talked about revoking birthright citizenship for years — including in 2015, when he was still a long-shot candidate. Were it to succeed, the proposal would alter something foundational in this country, and not for the better. It would create a permanent underclass of those born here but barred from claiming an American birthright.

In a land of immigrants, that would be shameful.