President-elect Donald Trump said in an interview with NBC’s Kristen Welker on an episode of Meet the Press that aired Sunday that he plans to abolish birthright in the United States, reaffirming a position he has previously taken had expressed.

If Trump tried to implement this plan after taking office, it would mean reversing the way the U.S. has handled citizenship for more than 150 years.

But can Trump end birthright citizenship in the U.S., and what happens if he does?

What did Trump say?

When Welker asked Trump if he still planned to abolish birthright on the first day of his term, Trump replied: “Yes, absolutely.”

Birthright citizenship essentially means that anyone born in the United States automatically becomes a U.S. citizen, which includes children of undocumented immigrants or tourists and students on short-term visas.

He said he was willing to work with Democrats to retain “Dreamers,” who are undocumented people who came to the U.S. as children and have lived there most of their lives.

However, Trump also said: “I don’t want to destroy families. The only way to keep the family from breaking up is to keep them together and have to send them all back.” That would mean deporting legal U.S. citizens – supposedly so that their families won’t be separated.

What does the 14th Amendment say?

The 14th Amendment to the U.S. Constitution is the foundation for the country’s approach to citizenship. It guarantees the birthright to citizenship. The amendment was ratified in 1868.

The first section of the amendment states: “All persons born or naturalized in the United States and subject to the jurisdiction therein are citizens of the United States and of the State in which they reside.”

This applies regardless of “the immigration or citizenship status of their parents,” according to the website of the American Immigration Council, a Washington, D.C.-based nonprofit advocacy group.

Can Trump end birthright citizenship?

During her conversation with Trump, Welker brought up the 14th Amendment and asked him if he could get around the law.

“Well, we have to change something,” Trump replied.

“But we have to end it.”

Trump said if he could, he would change it through executive action, among other options that he did not elaborate on.

Since his first term in office, Trump has talked about wanting to abolish birthright citizenship. In an interview with Axios in 2018, he said, “You can definitely do it with an act of Congress… but now they’re saying I can just do it with an executive order.”

Legal experts and politicians, including some in the Republican Party, have doubted Trump’s claims for years.

“You can’t end birthright citizenship with an executive order,” former Republican House Speaker Paul Ryan said in a 2018 radio interview. “I think in this case the 14th Amendment is pretty clear, and that would require a very, very lengthy constitutional process.”

Amending the U.S. Constitution is a difficult process. It requires two-thirds of the vote in both the House and Senate. The change must then be ratified by three-quarters of the state legislature. In the Senate, Democrats have 47 seats and Republicans have 53. In the House of Representatives, Democrats have 215 seats and Republicans have 220 seats.

What impact could this have?

A 2011 American Immigration Council fact sheet states that eliminating citizenship by birth will affect everyone. All American parents would have to go through a laborious and expensive process to prove their children’s citizenship.

“Our birth certificates are proof of our citizenship. If birthright citizenship were eliminated, U.S. citizens would no longer be able to use their birth certificates as proof of citizenship,” the fact sheet states.

Research from the Washington, D.C.-based nonpartisan think tank Migration Policy Institute and Pennsylvania State University estimates that eliminating birthright rights for U.S. children with two unauthorized immigrant parents will increase the existing “unauthorized population” by 4.7 million people by 2050 would increase. The results of this study were published in 2010.

Do other countries have birthright citizenship?

While Trump claimed in the NBC interview that the US is “the only country that has citizenship by birth,” more than 30 other countries are following this approach.

These are Antigua and Barbuda, Argentina, Barbados, Belize, Bolivia, Brazil, Canada, Chad, Chile, Costa Rica, Cuba, Dominica, Ecuador, El Salvador, Grenada, Guatemala, Guyana, Honduras, Jamaica, Lesotho, Mexico, Nicaragua, Panama, Paraguay, Peru, St. Kitts and Nevis, St. Lucia, St. Vincent and the Grenadines, Tanzania, Trinidad and Tobago, Tuvalu, Uruguay and Venezuela.

What other types of citizenship are there?

In many countries there is more than one way to grant citizenship. In addition to birthright citizenship, the following types of citizenship also exist in individual countries:

Citizenship by descent: This is the case when one or both parents must be citizens of a country for the child to be considered a citizen. Most countries offer citizenship by descent. To obtain citizenship by descent, it is not necessary to be born in the territory of the country. Some countries have specific criteria for granting citizenship based on descent. For example, in Bahrain and Iran, the father must be a citizen for the child to be considered a citizen.

Citizenship through naturalization: A foreign-born resident of a country can obtain citizenship after spending a certain amount of time there and meeting a set of criteria, usually evidence of having integrated into that culture. Different countries have different criteria for naturalization. The USA also offers this type of citizenship.

Citizenship by marriage: A person born abroad can obtain citizenship of a country if he or she is married to a citizen of that country. In the United States, people can apply for citizenship by marriage three years after marrying a U.S. citizen and receiving a permanent resident permit or green card. Regular residents must wait until five years after receiving their green card to apply for citizenship. Other countries that offer citizenship through marriage include the United Kingdom and Germany.

Dual citizenship: This means that a person can have citizenship of two countries. According to World Population Review, at least 87 countries allow dual citizenship with the United States.

Citizenship by Investment: In some countries, foreign-born residents can acquire citizenship through an investment in the country, such as purchasing property. In Turkey, foreigners must purchase real estate worth at least $400,000 to obtain citizenship. The USA does not have this option. Citizenship can also be acquired through investments in other countries, including Malta and Antigua and Barbuda.