US PRESIDENT Donald Trump is at war with Republican Paul Ryan over the issue of birthright citizenship, calling on the Speaker of the House to keep his opinions to himself because he “knows nothing”.

The remarkable comments from the president came after Mr Ryan commented on Mr Trump’s stated plan to issue an executive order that would seek to end the right to citizenship for children of noncitizens born in the United States.

“You obviously cannot do that,” Mr Ryan told Kentucky’s WVLK-AM, adding that the 14th Amendment to the United States Constitution “is pretty clear” on the issue.

Section 1 of the 14th Amendment reads: “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. No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.”

But Mr Trump has a different reading, tweeting today that “It is not covered by the 14th Amendment because of the words ‘subject to the jurisdiction thereof’,” he wrote. “This case will be settled by the United States Supreme Court.”

Mr Trump also lashed out at Mr Ryan saying he should be focusing on ensuring the Republicans hold its majority in both houses rather than commenting on the issue.

Mr Trump also pointed out that some Democrats, including the party’s leader in the Senate, Harry Reid, once agreed that birthright citizenship was a crazy idea.

However, Senator Reid said he had “evolved” on the issue since those comments.

“In 1993, around the time Donald Trump was gobbling up tax-free inheritance money from his wealthy father and driving several companies into bankruptcy, I made a mistake,” Senator Reid said in a statement. “And in my 36 years in Washington, there is no more valuable lesson I learned than the strength and power of immigrants and no issue and I worked harder on than fixing our broken immigration system,”

Mr Ryan and scholars widely pan the idea that Mr Trump could unilaterally change the rules on who is a citizen. And it’s highly questionable whether an act of Congress could do it, either.

Mr Trump has discussed the issue before and reinjected it into the political conversation just days before the 2018 midterms as he looks to energise his base.

Any change to the US constitution would need a two-thirds majority in both the House and the Senate.