DONALD Trump has a radical new idea to crack down on illegal immigrants.

At the moment, if the child of a non-citizen is born in the United States, that child is guaranteed American citizenship.

Mr Trump wants to remove that right.

He flagged the dramatic move in an interview with Axios today.

“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,” Mr Trump said.

“It’s ridiculous. It’s ridiculous. And it has to end.”

His claim that America is the “only country” with birthright citizenship is not correct. There are in fact 30 nations which grant it, including Canada, Argentina, Brazil and Mexico. Most of them are in Central and South America.

Don’t ask me why he said it. Like many Trumpian fibs, this one was weird and seemingly pointless.

However, putting that aside, the President’s idea to limit birthright citizenship is actually fairly mainstream.

Australia scrapped its own right as part of a broader citizenship overhaul in 2007. Britain, France and Germany top a long list of Western countries without one.

The thing that makes Mr Trump’s plan borderline insane is the fact that he can’t implement it. It’s probably a gigantic waste of time.

“It was always told to me that you needed a constitutional amendment. Guess what? You don’t,” Mr Trump said.

“You can definitely do it with an act of Congress. But now they’re saying I can do it just with an executive order.”

Yeah. He can’t.

The 14th amendment of America’s constitution, which was originally implemented to grant former slaves citizenship, says “all persons born or naturalised in the United States, and subject to the jurisdiction thereof, are citizens of the United States and the state wherein they reside”.

That statement was tested, and strengthened, by a Supreme Court ruling all the way back in 1898. There is more than a century of legal precedent supporting it.

If Mr Trump were to sign an executive order redefining birthright citizenship, it would undoubtedly be challenged in court. The same thing would happen if Congress passed legislation to the same effect.

In either case, he would lose.

“Sometimes the constitution’s text is plain as day and bars what politicians seek to do. That’s the case with President Trump’s proposal,” lawyers George Conway and Neal Katyal wrote for the Washington Post today.

Mr Conway is the husband of senior Trump adviser Kellyanne Conway.

“Such a move would be unconstitutional and would certainly be challenged. And the challengers would undoubtedly win,” they said.

“Our constitution is a bipartisan document, designed to endure for ages. Its words have meaning that cannot be wished away.”

Even leaders on Mr Trump’s own side of politics have grave doubts.

“You obviously cannot do that. You cannot end birthright citizenship with an executive order,” Republican House Speaker Paul Ryan said today.

Senator Ted Cruz, a former lawyer who has argued in front of the Supreme Court himself, said ending the right “makes a lot of common sense” but the courts would have to sort out the legal argument.

A few years ago, he was far blunter.

“The 14th Amendment provides for birthright citizenship. I’ve looked at the legal arguments against it and I will tell you, as a Supreme Court litigator, those arguments are not very good,” Mr Cruz said in 2011. “As much as someone may dislike the policy … it’s in the US constitution.”

There are some in Congress who enthusiastically support Mr Trump’s idea. Senator Lindsey Graham is among them.

“Finally, a president willing to take on this absurd policy of birthright citizenship,” Mr Graham said, signalling his intention to introduce legislation dealing with the issue.

According to the Migration Policy Institute, there are more than four million children under the age of 18 who live in the United States, were born there, and whose parents are illegal immigrants.

The White House genuinely believes it can deal with them. AP reports administration lawyers are planning to work with the Justice Department to develop a legal justification for an executive order.

“It’s in the process. It’ll happen, with an executive order,” Mr Trump told Axios.

But it seems the only way to actually accomplish Mr Trump’s policy would be to pass a new constitutional amendment. That is, to put it mildly, not an easy task.

First, a two-thirds majority in both houses of Congress would need to vote in favour of the proposed amendment. That is an impossible prospect without the support of at least some Democrats.

Then the 50 American states would vote on it. For the amendment to be added to the constitution, three-quarters of them would need to support it.

The Republican Party currently controls 31 state legislatures, and that number will probably fall after next week’s midterm elections. Again, if the Democrats oppose Mr Trump’s idea — and they certainly will — it will go nowhere.

So what’s the point? With the midterms just days away, a cynic might think Mr Trump had floated the policy for purely political reasons.

RELATED: The midterm elections explained

Mr Trump has made immigration a central issue of the campaign, stoking voters’ fears about illegal migrants. He has ordered 5200 National Guard troops to the Mexican border to “harden” it before the arrival of a caravan of migrants from Central America.

“Many gang members and some very bad people are mixed into the caravan,” the President said on Monday. “This is an invasion of our country and our military is waiting for you.”