Maryland joined 21 states, Washington, D.C., and San Francisco to sue President Trump on Tuesday over what they called his “flagrantly unlawful attempt” to end birthright citizenship through one of the flurry of executive orders he signed after taking office.

Birthright citizenship, which guarantees citizenship to children born in the U.S. regardless of their parents’ status, “has been enshrined in the Constitution for more than 150 years,” according to the suit filed in U.S. District Court in Massachusetts by the attorneys general, including Maryland’s Anthony Brown.

The state officials argue that Trump’s executive order violates the 14th Amendment, which was ratified after the Civil War and extended citizenship to formerly enslaved people.

“It is a reflection of our country’s ideals, a belief that every baby born on U.S. soil is a member of our great nation and deserves to play a part in its future,” Brown said in a prepared statement. “Ending birthright citizenship is un-American, and our office will vigorously challenge this blatantly unconstitutional decision in court.”

The suit seeks to invalidate Trump’s executive order and to prevent it from taking place.

Denying birthright citizenship would strip hundreds of thousands of individuals of “their most basic rights,” Brown’s statement said, forcing them to live under threat of deportation and, as the children age, the ability to work lawfully, vote and run for office.

Trump’s order states babies born after Feb. 19 must have at least one parent who is a citizen or a lawful permanent resident in order to become citizens themselves.

The ACLU and immigrant rights groups have also sued Trump over the order, calling it “a reckless and ruthless repudiation of American values.”

Maryland Sen. Justin Ready, a Republican who represents Carroll and Frederick counties, said rather than engage in “showboat lawsuits,” statewide leaders should try to find a way to work with Trump even if they disagree with him.

“It seems like if you’re a blue-state Attorney General, you have to sue Trump,” Ready said.

He said birthright citizenship has exceptions, such as for the children of diplomats, and “it’s not an infallible right.”

Martha Jones, a Johns Hopkins history professor and author of the 2018 book, “Birthright Citizens: A History of Race and Rights in Antebellum America, said Trump’s executive order is “a perversion” of an amendment that extended and made citizenship more inclusive.

“We are opening up a harrowing chapter where people don’t know how these changes will apply to them, or where they will end,” said Jones a professor at Hopkins’ SNF Agora Institute.

Birthright citizenship has protected many Americans over the years, from World War II to the McCarthy era to post-911, when entire groups could have been “de-naturalized” because they were perceived as enemies at the time, she said.

The issue is particularly resonant in Baltimore, which played a decisive role in the concept gaining traction and eventually becoming part of the Constitution, Jones said. The city had the country’s largest group of free Black residents in the 1800, and activists fought hard to have their right to citizenship codified amid threats that they could face either being removed from the country or even re-enslaved.

The ratification of the 14th Amendment in 1868 overturned the Supreme Court’s infamous Dred Scott decision, which denied citizenship to Black people, and was written by Roger Taney of Maryland.

“It was a long legal struggle and an ending that comports with what Black activists from Baltimore had been calling for for decades,” Jones said.

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