WASHINGTON (Reuters) - President Donald Trump said on Wednesday the U.S. Constitution does not guarantee the right to citizenship to everyone born in the country, an assertion that runs counter to the long-established legal interpretation of the document.

“So-called Birthright Citizenship, which costs our Country billions of dollars and is very unfair to our citizens, will be ended one way or the other. It is not covered by the 14th Amendment because of the words ‘subject to the jurisdiction thereof.’ Many legal scholars agree.....” Trump wrote in a Twitter post six days before U.S. congressional elections.

The Constitution’s 14th Amendment, added after the Civil War, grants citizenship to anyone born on American soil and was intended to give constitutional protections to former slaves. But some Republicans, including Trump, say it creates an incentive for people to enter the country illegally to have children.

George Conway, the lawyer husband of one of Trump’s top advisers, Kellyanne Conway, wrote in an opinion piece in the Washington Post on Wednesday that such a move to end birthright citizenship would be unconstitutional.

“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 to end ‘birthright citizenship’ through an executive order,” the attorney wrote.

White House Press Secretary Sarah Sanders said in an interview with Fox News on Wednesday there was “more than just one piece of the puzzle” on immigration.

“The president wants to see a total reform take place. We have massive loopholes in our immigration system that we have to close or we’re just going to continue kicking the can down the road,” she added.

Trump first raised the issue of scrapping birthright citizenship on Tuesday. The proposal gained support from Republican Senator Lindsey Graham, a Trump ally. But other party members were critical, including House of Representatives Speaker Paul Ryan, who said the president could not end the right with the stroke of a pen.

Trump - who has made rhetoric against illegal immigrants and policies to curb both illegal and legal immigration key themes of his presidency - has raised it in rallies ahead of the Nov. 6 election, in which Republicans are trying to maintain control of Congress in the face of challenges by Democrats.