A federal judge has temporarily blocked President Trump's executive order ending birthright citizenship, setting up the first major legal battle of the new Trump era.

The ruling from Judge John Coughenour, a Ronald Reagan appointee, presents a test for the president as he fights to deliver on his mandate of sweeping immigration reform.

The judge dismissed Trump's action as "blatantly unconstitutional" and questioned the legal training of the president's lawyers during a hearing Thursday.

“This is a blatantly unconstitutional order,” Coughenour said. “I’ve been on the bench for over four decades, I can’t remember another case where the question presented is as clear as this one is,” he added.

Trump's EO blocked

The judge issued a temporary restraining order, blocking the order from being enforced, nationwide, for 14 days.

If enforced, Trump's order would end the practice of automatically granting citizenship to anyone born on U.S. soil - specifically, the children of illegal aliens and visa holders.

Trump's order sparked a quick legal challenge from over 20 Democratic states, which argued birthright citizenship is guaranteed by the Constitution.

The relevant text 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.”

While the language has long been interpreted to grant automatic citizenship to anybody born in the U.S., critics of birthright citizenship say this understanding misrepresents the intent of the 14th Amendment, which was ratified to guarantee the citizenship of former slaves.

Some argue that the phrase "subject to the jurisdiction thereof" was meant to include only those individuals who have no allegiances to any foreign sovereign - which would exclude illegal immigrants.

Dispute over "jurisdiction"

During Thursday's hearing, Justice Department lawyer Brett Shumate argued that illegal aliens, while subject to prosecution for breaking U.S. law, "remain subject to a foreign power” and therefore “have no allegiance to the United States."

But Judge Coughenour made it clear that the issue has long been settled, in his view.

"Frankly, I have difficulty understanding how a member of the Bar could state unequivocally that this is a constitutional order,” Coughenour said. "It just boggles my mind.”

The Justice Department, however, is not backing down.

"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.

It appears likely that this dispute will end up in front of the Supreme Court. While this is a setback for Trump, it's in his best interest that this issue is being confronted early on in his term.