A federal judge on Thursday temporarily blocked President Donald Trump’s executive order denying U.S. citizenship to the children of parents living in the country illegally, calling it “blatantly unconstitutional” during the first hearing in a multi-state effort challenging the order.

The 14th Amendment to the Constitution promises citizenship to those born on U.S. soil, a measure ratified in 1868 to ensure citizenship for former slaves after the Civil War. But in an effort to curb unlawful immigration, Trump issued the executive order just after being sworn in for his second term on Monday.

The order would deny citizenship to those born after Feb. 19 whose parents are in the country illegally. It also forbids U.S. agencies from issuing any document or accepting any state document recognizing citizenship for such children.

Trump's order drew immediate legal challenges across the country, with at least five lawsuits being brought by 22 states and a number of immigrants rights groups. A lawsuit brought by Washington, Arizona, Oregon and Illinois was the first to get a hearing.

"I’ve been on the bench for over four decades. I can’t remember another case where the question presented was as clear as this one is,” U.S. District Judge John Coughenour told a Justice Department attorney. “This is a blatantly unconstitutional order.”

Thursday’s decision prevents the Trump administration from taking steps to implement the executive order for 14 days. In the meantime, the parties will submit further arguments about the merits of Trump's order. Coughenour scheduled a hearing on Feb. 6 to decide whether to block it long term as the case proceeds.

Coughenour, 84, a Ronald Reagan appointee who was nominated to the federal bench in 1981, grilled the DOJ attorney, Brett Shumate, asking whether Shumate personally believed the order was constitutional.

“I have difficulty understanding how a member of the bar could state unequivocally that this is a constitutional order,” he added.

Shumate assured the judge he did — “absolutely.” He said the arguments the Trump administration is making now have never previously been litigated, and that there was no reason to issue a 14-day temporary restraining order when it would expire before the executive order takes effect.

The Department of Justice later said in a statement that it will “vigorously defend” the president’s executive order, which it said “correctly interprets the 14th Amendment of the U.S. Constitution.”

“We look forward to presenting a full merits argument to the Court and to the American people, who are desperate to see our Nation’s laws enforced,” the department said.

Read: Trump signs executive order to end birthright citizenship

The U.S. is among about 30 countries where birthright citizenship — the principle of jus soli or “right of the soil” — is applied. Most are in the Americas, and Canada and Mexico are among them.

The 14th Amendment was ratified in 1868, in the aftermath of the Civil War, to ensure citizenship for former slaves and free African Americans. It states: “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 asserts that the children of noncitizens are not “subject to the jurisdiction” of the United States, and therefore not entitled to citizenship.

Arguing for the states on Thursday, Washington assistant attorney general Lane Polozola called that “absurd," noting that neither those who have immigrated illegally nor their children are immune from U.S. law.

“Are they not subject to the decisions of the immigration courts?” Polozola asked. “Must they not follow the law while they are here?”

Polozola also said the restraining order was warranted because, among other reasons, the executive order would immediately start requiring the states to spend millions to revamp health care and benefits systems to reconsider an applicant’s citizenship status.

“The executive order will impact hundreds of thousands of citizens nationwide who will lose their citizenship under this new rule,” Polozola said. “Births cannot be paused while the court considers this case.”

Washington Attorney General Nick Brown told reporters afterward he was not surprised that Coughenour had little patience with the Justice Department’s position, considering that the Citizenship Clause arose from one of the darkest chapters of American law, the Supreme Court’s 1857 Dred Scott decision, which held that African Americans, whether enslaved or free, were not entitled to citizenship.

Read more: Trump says birthright citizenship will be ended

“Babies are being born today, tomorrow, every day, all across this country, and so we had to act now,” Brown said. He added that it has been “the law of the land for generations, that you are an American citizen if you are born on American soil, period.”

“Nothing that the president can do will change that,” he said.

A key case involving birthright citizenship unfolded in 1898. The Supreme 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 being 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 case clearly applied to children born to parents who were both legal immigrants. They say it’s less clear whether it applies to children born to parents living in the country illegally.

Trump’s order prompted attorneys general to share their personal connections to birthright citizenship. Connecticut Attorney General William Tong, for instance, a U.S. citizen by birthright and the nation’s first Chinese American elected attorney general, said the lawsuit was personal for him. Later Thursday, he said Coughenour made the right decision.

“There is no legitimate legal debate on this question. But the fact that Trump is dead wrong will not prevent him from inflicting serious harm right now on American families like my own,” Tong said this week.