Attorneys general from 18 states, including Colorado, sued Tuesday to block President Donald Trump’s move to end a decades-old immigration policy known as “birthright citizenship,” which guarantees that U.S.-born children are citizens regardless of their parents’ status.

Trump’s roughly 700-word executive order, issued late Monday, amounts to a fulfillment of something he’s talked about during the presidential campaign. But whether it succeeds is far from certain amid what is likely to be a lengthy legal battle over the president’s immigration policies.

“The White House executive order attempting to end birthright citizenship is flatly unconstitutional,” said Colorado Attorney General Phil Weiser. “The idea that a president could override the Constitution with the stoke of a pen is a flagrant assault on the rule of law and our constitutional republic.”

The Trump administration said the Fourteenth Amendment has never been interpreted to “extend citizenship universally to everyone born within the United States.”

“The Fourteenth Amendment has always excluded from birthright citizenship persons who were born in the United States but not ‘subject to the jurisdiction thereof,'” the administration said. 

Here’s a closer look at birthright citizenship, Trump’s executive order and reaction to it:

What is birthright citizenship?

Birthright citizenship means anyone born in the U.S. is a citizen, regardless of their parents’ immigration status. People, for instance, in the United States on a tourist or other visa or in the country illegally can become the parents of a citizen if their child is born here.

It’s been in place for decades and enshrined in the 14th Amendment to the Constitution, supporters say. But Trump and allies dispute that reading of the amendment and argue there need to be tougher standards on becoming a citizen.

What does Trump’s order say?

The order says the 14th Amendment does not extend citizenship automatically to anyone born in the United States.

The 14th Amendment was born in the aftermath of the Civil War and ratified in 1868. It says: “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.”

Trump’s order excludes the following people from automatic citizenship: individuals whose mothers were not legally in the United States and whose fathers were not U.S. citizens or lawful permanent residents; people whose mothers were in the country legally but on a temporary basis — such as on a  tourist visa — and whose fathers were not citizens or legal permanent residents.

It goes on to bar federal agencies from recognizing the citizenship of people in those categories. It takes effect 30 days from Tuesday, on Feb. 19.

What is the history of the issue?

The 14th Amendment did not always guarantee birthright citizenship to all U.S.-born people. Congress did not authorize citizenship for all Native Americans born in the United States, for instance, until 1924.

In 1898, an important birthright citizenship case unfolded in the U.S. Supreme Court. The court held that Wong Kim Ark, who was born in San Francisco to Chinese immigrants, was a U.S. citizen because he was born in the country. After a trip abroad, he had faced denied reentry by the federal government on the grounds that he wasn’t a citizen under the Chinese Exclusion Act.

But some advocates of immigration restrictions have argued that while the case clearly applied to children born to parents who are both legal immigrants, it’s less clear whether it applies to children born to parents without legal status.

What has the reaction to Trump’s order been?

Eighteen states, including Colorado, plus the District of Columbia and San Francisco, sued in federal court to block Trump’s order.

New Jersey Democratic Attorney General Matt Platkin said Tuesday that presidents don’t have the authority to undo a constitutional provision. 

Not long after Trump signed the order, immigrant advocacy groups also filed suit to stop it.

Chapters of the American Civil Liberties Union in New Hampshire, Maine and Massachusetts, along with other advocates, filed a suit in New Hampshire federal court.

The suit asks the court to find the order to be unconstitutional. It highlights the case of a woman identified as “Carmen,” who is pregnant but is not a citizen. The lawsuit says she has lived in the United States for more than 15 years and has a pending visa application that could lead to permanent status. She has no other immigration status, and the father of her expected child has no immigration status either, the suit says.

In addition to New Jersey and the two cities, California, Massachusetts, Colorado, Connecticut, Delaware, Hawaii, Maine, Maryland, Michigan, Minnesota, Nevada, New Mexico, New York, North Carolina, Rhode Island, Vermont, and Wisconsin joined the lawsuit to stop the order.

Associated Press reporter David Collins in Hartford, Connecticut, contributed to this report.