WASHINGTON — Teeing up a blockbuster ruling, the Supreme Court on Friday agreed to decide the lawfulness of President Donald Trump’s contentious plan to roll back automatic birthright citizenship for nearly anyone born in the United States.

The eventual ruling in a case from New Hampshire, expected by the end of June, will likely determine conclusively whether Trump’s ambitious proposal can move forward.

The case sets up a major clash between a president whose aggressive use of executive power has been a defining characteristic of his second term and a court with a 6-3 conservative majority that has so far mostly avoided direct clashes with the White House.

Birthright citizenship has long been understood to be required under the Constitution’s 14th Amendment, which states: “All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States.”

The language was included in the constitutional amendment enacted after the Civil War to ensure that Black former slaves and their children were recognized as citizens.

Legal scholars of all ideological stripes have generally assumed the phrase to be self-explanatory, with the only exceptions being people born to foreign diplomats, invading hostile forces and members of some Native American tribes.

But Trump, as part of his immigration crackdown, has sought to unravel that historical understanding, embracing a hitherto fringe theory pushed by anti-immigration activists.

Under the administration’s view, birthright citizenship would be limited to those who have at least one parent who is a U.S. citizen or permanent legal resident. In that scenario, the right would not apply to babies born to temporary visitors who entered the country legally or to people who entered the country illegally.

The administration’s legal argument, presented by Solicitor General D. John Sauer, is that the “subject to the jurisdiction thereof” language confers citizenship upon only children who are not just present in the United States but also bear allegiance to it.

It is not enough merely to be subject to U.S. law, which is how the clause has traditionally been interpreted, he argues.

“Yet, long after the Clause’s adoption, the mistaken view that birth on U.S. territory confers citizenship on anyone subject to the regulatory reach of U.S. law became pervasive, with destructive consequences,” Sauer wrote in court papers.

Trump’s executive order is an effort to “restore the Clause’s original meaning,” he added.

The case involves individual plaintiffs represented by the American Civil Liberties Union. The plaintiffs, who use pseudonyms, include two babies who would be subject to the order.

Trump’s executive order runs “squarely contrary to the constitutional text, this court’s precedents, Congress’s dictates, longstanding Executive Branch practice, scholarly consensus, and well over a century of our nation’s everyday practice,” the challengers’ lawyers wrote in court papers.

They point to an 1898 case, called United States v. Wong Kim Ark, in which the Supreme Court ruled that a man born in San Francisco to parents who were both from China was a U.S. citizen.

Announced on Trump’s first day in office, on Jan. 20, the policy has fared badly in lower courts, with judges across the nation ruling it unlawful, as they did in the New Hampshire case. As a result, the plan has not been implemented.

The Trump administration already secured Supreme Court intervention when it successfully argued that individual judges did not have the authority to block the plan nationwide. That decision did not, however, touch upon the legal merits of the executive order.

The court did not act on a separate case concerning a lawsuit brought by the states of Washington, Arizona, Illinois and Oregon.

The Supreme Court has been generally receptive to the Trump administration in cases brought this year, but many legal observers believe the birthright citizenship showdown may be an exception.

Major tests of Trump’s executive power are now piling up at the court. A ruling on the president’s broad use of tariffs is due at any time, while the justices are also set to weigh in on his authority to fire members of executive government entities, including the Federal Reserve, in the coming months.