The federal judge reignited the legal standoff over President Trump’s efforts to deny citizenship to children born to undocumented parents.

A federal judge on Thursday blocked the Trump administration from enforcing a contentious executive order ending birthright citizenship after certifying a lawsuit as a class action, effectively the only way he could impose such a far-reaching limit after a Supreme Court ruling last month.

Ruling from the bench, Judge Joseph N. Laplante of the U.S. District Court for the District of New Hampshire said his decision applied nationwide to babies who would have been subject to the executive order, which included the children of undocumented parents and those born to academics in the United States on student visas, on or after Feb. 20.

The Trump administration has fought to challenge the longstanding law, laid out in the Constitution, that people born in the United States are automatically citizens, regardless of their parents’ immigration status. Judge Laplante’s order reignites a legal standoff that has been underway since the beginning of President Trump’s second term.

The judge, an appointee of President George W. Bush, issued a written order formalizing the ruling on Thursday morning. He also paused his order for seven days, allowing time for an appeal.

The lawsuit, brought by the A.C.L.U., was filed hours after the Supreme Court’s ruling last month limiting the ability of lower court judges to issue nationwide injunctions. The case tested the new landscape after the Supreme Court decision and made use of what appeared to be the only remaining practical and efficient way for district court judges to freeze the implementation of policies they found unlawful.

Class actions, which involve a population of similarly situated people, were seen as the workaround. Before the Supreme Court’s decision, nationwide or “universal” injunctions were the main tool judges could use to halt executive branch policies.

The Supreme Court’s decision on birthright citizenship did not address the core dispute surrounding the constitutionality of Mr. Trump’s executive action, but it paved the way for a majority of states to begin enforcing it. However, the court’s majority said the executive order could not be carried out until July 27, allowing time for lawsuits to be filed.

The A.C.L.U.’s lawsuit had proposed that all children born in the United States after Feb. 20 and their parents constituted a class. It warned that under the terms of Mr. Trump’s order, people born to parents in the country unlawfully risked being rendered “effectively stateless.”

“For families across America today, birthright citizenship represents the promise that their children can achieve their full potential as Americans,” the lawsuit said. “It means children born here can dream of becoming doctors, lawyers, teachers, entrepreneurs or even president — dreams that would be foreclosed if their citizenship were stripped away based on their parents’ status.”

Cody Wofsy, the deputy director of the A.C.L.U. Immigrants’ Rights Project, said on Thursday that the judge ultimately limited the class to newborns, not their parents.

But the judge indicated that he had few reservations about blocking the policy as it applied to children who would end up in limbo, saying his decision was “not a close call.”

Tom Homan, Mr. Trump’s border czar, declined to say how the administration would respond legally to the ruling. But, following a well-worn pattern for the Trump administration, he lit into the judge.

“It’s another example of a radical judge not paying attention to the Supreme Court ruling,” he said, adding: “These radical judges aren’t going to stop. They want to stop the Trump agenda. And what they need to understand: The American people voted for this agenda.”

Birthright citizenship has its origins in English common law and was enshrined in the U.S. Constitution through the 14th Amendment.

The amendment holds that “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.”

Courts have consistently rejected the idea, put forth in Mr. Trump’s order, that children born to undocumented immigrants should not be covered by the 14th Amendment. Many constitutional law experts believe that the order is unconstitutional on its face.

In his order, Judge Laplante said that Mr. Trump’s executive action, which would deny citizenship to children already born this year and in the future, clearly harmed the class he certified.

He noted that he decided to block implementation of the executive order because the harm faced by the people who brought the lawsuit far outweighed any potential harm to the government, and that the plaintiffs would likely succeed in arguing that Mr. Trump’s order was unconstitutional.

Judge Laplante said citizenship was the most foundational benefit the government could extend.

“It is the greatest privilege that exists in the world,” he said.

The A.C.L.U., which has been among the leading groups fighting the Trump administration’s immigration policies, celebrated the ruling and its success in bringing a class-action challenge as it navigates the recent Supreme Court decision.

“This ruling is a huge victory and will help protect the citizenship of all children born in the United States, as the Constitution intended,” Mr. Wofsy said in a statement. “We are fighting to ensure President Trump doesn’t trample on the citizenship rights of one single child.”

Without the lawsuit, immigration advocacy groups had warned of a chaotic legal patchwork after July 27, when the country would fracture into regions where the president’s order had been blocked and others where it would take effect.

In their dissent in the Supreme Court’s decision in June, the court’s liberal members also warned of an endlessly confusing environment in which people or groups could be forced to file individual lawsuits to advocate a child’s citizenship as each baby is born.