Trump has not offered a time frame for when the order would be advanced. It is unclear what specific actions would be outlined.

Birthright citizenship is outlined in the 14th Amendment. An executive order to revoke that right for millions of U.S. citizens would rattle the Constitution and likely prompt immediately legal challenges and opposition in Congress.

Under the 14th Amendment, "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."

Changing an amendment in the Constitution requires the endorsement of two-thirds of the U.S. House of Representatives and the Senate and the backing of three-fourths of U.S. state legislatures at a constitutional convention, Reuters says.

However, Trump has said that he has talked to his legal counsel and has been advised that he would be able to enact the change unilaterally. This is contrary to many constitutional experts’ interpretations of the law.

"You can definitely do it with an act of Congress. But now they're saying I can do it just with an executive order," Trump said. "It's in the process. It'll happen."

U.S. policies on immigration have been constitutionally questionable since Trump has taken office. Earlier this year, family separations at the U.S. / Mexico border left more than 2,000 children detained without their parents in I.C.E.  (Immigration and Customs Enforcement) facilities.

Shortly after his inauguration in 2017, he signed an order banning travel to the U.S. for people from multiple Muslim-majority countries. Doctors working in rural areas, students, and a young girl in need of medical treatment were among those affected.

During his presidential campaign, Trump referred to children who are born in the U.S. to undocumented parents as ‘anchor babies.’ The term is derogatory and misleading.

U.S. law does not grant residency or citizenship to parents of children born in the U.S. The children must wait until they are 21 to petition for their parent’s legal status. There are very few exceptions to this rule.

In 2012, it was estimated that 4.5 billion people were birthright citizens in the U.S. based on data from the Census Bureau.

His comment comes a week before the U.S. midterm elections, and just after 5,200 troops have been deployed to the U.S. / Mexico border in anticipation of the migrant caravan fleeing violence and poverty in Honduras and other Central American countries.

It also comes a week after a surge in violence around the country, including a series of mailed explosive devices sent to high-profile Democrats and a violent attack on a synagogue. The shooting in Pittsburgh, Pennsylvania, is the deadliest attack on a synagogue in U.S. history.

A legal shift of this magnitude would prompt the nation's courts to weigh in. It would be one of the most sweeping moves of the Trump administration.