President Donald Trump has said he is preparing an executive order that will nullify the long-accepted constitutional guarantee of birthright citizenship in the US, his latest attention-grabbing manoeuvre days before midterm congressional elections as he sought to activate his base by vowing to clamp down on immigrants and immigration.

“We’re the only country in the world where a person comes in and has a baby, and the baby is essentially a citizen of the United States for 85 years, with all of those benefits,” Trump told Axios, the news and information website, during an interview that was released in part on Tuesday, making a false claim. “It’s ridiculous. It’s ridiculous. And it has to end.”

At least 30 other countries, including Canada, Mexico and many others in the Western Hemisphere, grant automatic birthright citizenship.

(In India, the law granting birthright citizenship was changed in 1987. Now, citizenship by birth is granted only to children at least one of whose parents is an Indian citizen and the other not an illegal immigrant.)

Trump’s plan met with swift pushback from some even in his own party on Tuesday. House Speaker Paul D. Ryan of Wisconsin, who is retiring, said in an interview that the President “obviously” could not eviscerate birthright citizenship by executive order.

“You obviously cannot do that,” Ryan told WVLK, a radio station in Lexington, Kentucky. “I’m a believer in following the plain text of the Constitution, and I think in this case, the 14th Amendment is pretty clear, and that would involve a very, very lengthy constitutional process.”

Doing away with birthright citizenship for the children of undocumented immigrants was an idea Trump pitched as a presidential candidate, but there is no clear indication that he would be able to do so unilaterally, and attempting to would be certain to prompt legal challenges. The consensus among legal scholars is that he cannot, but Trump and his allies are eager to test it in the Supreme Court.

“We all cherish the language of the 14th Amendment, but the Supreme Court of the United States has never ruled on whether the language of the 14th Amendment — ‘subject to the jurisdiction thereof’ — applies specifically to people who are in the country illegally,” Vice-President Mike Pence told Politico in an interview on Tuesday, several hours after Trump’s comments were reported.

To accomplish the idea he floated on Tuesday, Trump would have to find a way around the 14th Amendment to the Constitution, which states: “All persons born or naturalised in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.”

The amendment means that any child born in the US is considered a citizen. Amendments to the Constitution cannot be overridden by presidential action — they can be changed or undone only by overwhelming majorities in Congress or the states, with a two-thirds vote of both Houses of Congress or through a constitutional convention called for by two-thirds of state legislatures.